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This article will discuss in-depth combat strategy and military tactics in Civilization V. It is written for the Brave New World expansion, although some elements of it might serve for older versions of the game.
Introduction[]
The combat system in Civilization V and its expansion sets is elaborate and detailed, allowing for some really brilliant and quite lifelike military strategies to develop. Besides the individual features of generic units, we also have the promotion system which allows each individual unit to acquire additional strengths and turn better in some aspects of warfare. And after that we have the civilization-specific units, which have even more special abilities! All of this together creates a very interesting and quite sophisticated combat experience. In order to master the combat system of Civilization V, you will need to learn a lot, and hopefully this article will help you with that.
This article assumes that you're familiar with the basic terms used herein, such as 
Combat Strength (CS), Movement Points (MP), and so on, as well as the general rules for the types of combat (melee, ranged, etc.). If not, you may first want to read the other articles in the Combat section before starting this one.
Classes of military units[]
The first and foremost key to success in combat is to know the advantages and disadvantages of your army. For that, you must know what each particular class of unit is good at and what its drawbacks are, or you will never be able to form a clear strategy for its use. Civilization V uses a very sophisticated combat system, and in order to be successful in it you have to arm yourself not only with soldiers, but also with knowledge. Read the following breakdown and think when and how to use each unit you build: a correct use will take advantage of all its special abilities (for example, engage enemy cavalry with anti-cavalry units, such as Spearmen).
The following section won't discuss each individual unit - we have separate pages for that. But it will give you a breakdown of the strategic uses of the broad classes of units as a whole. Each of these has particular strengths and weaknesses that will be described here. Several main categories of features will be discussed: a unit's Combat Strength and/or 
Ranged Strength, movement abilities and special abilities. In some cases, where relevant, scouting abilities (or lack thereof) will also be emphasized.
Land units[]
Infantry[]
Types: Warrior, Swordsman, Longswordsman, Musketman, Rifleman, Great War Infantry, Infantry, Marine, Paratrooper, Mechanized Infantry, XCOM Squad (
only)
Melee and gunpowder units are collectively referred to as infantry. These are the units which form the backbone of your army. In early eras they are armed with melee weapons like axes, swords, and spears; in later eras they start using firearms. Regardless of their weaponry, they act similarly and serve a similar purpose throughout the ages: to soak up incoming damage and protect the more fragile parts of your army. That is why they're also called "front-line" units - because they are generally found in the first line of your battle order, exposed to all sorts of dangers and protecting other, more fragile units from harm. Before the Renaissance Era, infantry units use 
Iron.
Front-line units generally have higher CS than other units, which allows them to suffer relatively less damage. They can use the features of the terrain for defense and their main promotions are related to terrain (which makes the choice of where they will fight important) and to defense (primarily from ranged attacks). But they're always melee fighters, meaning that they can't attack with impunity; also they generally move slowly (2 MPs) and are otherwise quite inflexible.
Infantry are quite good at delivering the coup-de-grace in city sieges, as well as soaking up damage during the siege. They are the only class of units besides siege weapons and bomber units that can receive the Siege promotion, increasing their CS against cities; that promotion, however, comes quite late and requires a lot of experience. Remember that a city may only be taken by a melee unit, such as a front-line infantry unit. At the same time, remember that the city is quite a formidable adversary and will inflict severe damage to any unit attacking in melee, so don't be too eager to do that, or you will lose many units trying to take the city.
Note that all terrain-related promotions apply to the tile which is currently being attacked. This means that the defending unit will use promotions related to the tile being attacked, while the attacking unit will use promotions for the tile it's attacking, NOT the one it's currently occupying. It is a general mistake to use a unit with rough terrain promotions while attacking a flat terrain tile, then wonder why your bonuses didn't match up. Another very important factor is Formation bonuses, which mainly apply in melee combat, so consider carefully how you position your melee units along the front line! (Read below for more strategy on that.)
Ranged units[]
Types: Archer, Composite Bowman (
only), Crossbowman, Gatling Gun ( only), Machine Gun, Bazooka ( only)
Ranged units from all eras are sometimes called archers. These land units are frequently your main damage-dealers. However, they act quite differently in pre-Industrial and post-Industrial Eras, as detailed below.
Pre-Industrial Era: Early ranged units have long range (2 tiles) and considerable RS, but lower CS. As a result, they are very dangerous when attacking, because they can reach enemy units from afar and the enemy doesn't get to hurt them in turn. Several ranged units in good positions may inflict severe damage on the enemy army before it can even react. However, they are very weak when attacked by other units, and the more powerful melee units can sometimes wipe them out with a single attack. So, take good care to protect them with melee units and keep them away from the front line at all times.
Another important factor in early ranged combat is line-of-sight. Ranged units can't use their full Range of 2 if certain objects block their view, and since it is full of such terrain features, you will often find yourself in situations where your archers can't shoot at their full range because of terrain. To circumvent this, always look for "vantage points" (hills) to position your ranged units - this way they'll be able to use their full range, while also being well-defended by the terrain's features.
Post-Industrial Era: Modern ranged units have shorter range (1 tile), but their RS and CS are equal. This means that they're almost as good in defense as they are in offense, unlike earlier ranged units. Their main benefit comes from the fact that the enemy still can't retaliate to their attacks.
Positioning modern ranged units also changes - they now go to the front line of your army, since they won't ever be able to attack otherwise.
All ranged units move slowly (2 
Movement) and have normal sight range (2 tiles). Both pre- and post-Industrial Era shooters share a special line of promotions which is concentrated more on damage-dealing; their top-line promotions include the ability to attack twice per turn and the capability to heal while acting.
Mobile units[]
Types: Chariot Archer, Horseman, Knight, Cavalry, Landship ( only), Tank, Modern Armor, Giant Death Robot
Mounted units are sometimes called cavalry, and armored units are sometimes called tanks; mounted and armored units are collectively referred to as mobile units. These are special, highly maneuverable units designed to deliver concentrated damage to weak links in the enemy lines. The early mounted units use 
Horses to move fast, while the later armored units are heavy but fast combat vehicles, using 
Oil and later 
Aluminum. Both share extreme mobility (4 Movement, 5 for late armored units) and the ability to move after attacking, which allows them to escape retaliatory attacks and position themselves better on the battlefield.
With the exception of the Chariot Archer and its replacements, they are always melee units (meaning they also suffer damage when attacking), but their high CS generally gives them an edge over their targets. Also, they don't stick around to receive successive counter-strikes! A cavalry unit may swoop in and utterly destroy an enemy archer or siege unit, then escape before they know what hit them!
On the downside, both mounted and armored units can't use defensive terrain bonuses, which makes them relatively weaker when defending. Also, mounted units are quite bad at attacking cities. This isn't true for armored units, which may be used in sieges the same as infantry. These qualities make cavalry almost exclusively an offensive weapon, so be sure to use it accordingly.
Both mounted and armored units' promotions are related to terrain and attacking efficiency. But they also have an easy access to Scouting promotions, which makes them excellent recon units, especially in the early game and midgame.
Anti-mounted/Anti-tank units[]
Types: Spearman, Pikeman, Lancer, Anti-Tank Gun, Helicopter Gunship
This class of units is the most diverse, because they start as another type of infantry, then change several times until reaching their most-advanced modern unit, the Helicopter Gunship. The purpose of this class of units is to counter cavalry/armored units, and all of them have a bonus against the relevant unit. Note that the Bonus vs Mounted won't work against armored units, and vice versa! So, keep your units updated or you might find yourself in quite a pickle in the Industrial/Modern Era.
The other qualities of this class are diverse - check the individual units' descriptions for more info. Some of them are purely defensive in nature, such as the Pikeman (which is basically just another type of infantry); others, like the Lancer, are meant to attack/hunt down the enemy, depending on speed rather than endurance. They share one purpose, however: to defend your army from the above-mentioned classes of enemies, and you should always have some of them readily available.
Siege units[]
Types: Catapult, Trebuchet, Cannon, Artillery, Rocket Artillery
This class of units have one purpose: to reduce cities' defenses to rubble. They are basically ranged units (with Range 2) which are strong against cities (having a special CS bonus) and weak against pretty much everything else. They move slowly (2 MPs) and can't even fire without setting up first (which costs 1 MP); that means they can't get in range of a city and fire in the same turn! Furthermore, they can't use defensive terrain, have much lower CS when defending and finally have a limited sight radius (1 tile), which effectively makes them unusable if they're on their own. This means that you should always move them with your main army, not on their own, if you don't want to lose them to, say, a lone Horseman.
In the Industrial Era, these units increase their Range to 3. This is a real game changer, because they are now able to safely shoot at cities from outside of their defensive perimeter! Additionally, the most modern unit of the chain, the Rocket Artillery, doesn't need to set up in order to fire (but it still suffers from the other drawbacks).
Using siege weapons is quite tricky - they're not only slow and vulnerable, but they're also quite expensive to build. On the other hand, cities are really tough targets to take down, so you need siege weapons to attack them, even in the Modern Era. Check the siege section for some tactics on taking cities.
[]
Melee ships[]
Types: Trireme, Caravel, Privateer ( only), Ironclad, Destroyer
These ships are armed for naval melee combat and can only attack other ships in melee, or coastal cities. However, they are also the only ships capable of taking cities. They generally move fast (4 or more MP) and serve as a sort of "first line of defense" to your navy. The last ship in that chain, the Destroyer, is vital for protecting contemporary navies from submarine and aerial dangers - always have some of these!
Melee ships have special promotions specializing them in attacking other ships, or cities - choose one to unlock more powerful abilities such as a double attack. They can also increase their sight range with a relatively early promotion.
Ranged ships[]
Types: Trireme (vanilla only), Galleass ( only), Frigate, Battleship, Missile Cruiser
These ships don't start appearing until the Medieval Era, but they're very important tactically, because their ranged attack with a range of 2 can reach not only other ships, but also targets on land! During the Renaissance Era and especially in Modern and later Eras, when ships' attack range increases to 3, they could become major auxiliaries of your land campaigns, bombarding enemy units on land and coastal cities outside of their defensive perimeter. Regardless of the Era, they are always very effective against other ships. Ranged ships are also very mobile, although their speed compared to melee ships varies throughout the Eras - sometimes they're slower, sometimes faster.
Similar to Land units, naval ranged units have lower CS compared to their RS, so they're more vulnerable in defense - try to always escort them with melee ships for additional defense. Their promotions concentrate on either attacking Land or Naval units; they can also increase their Range by 1.
Submarines[]
Types: Submarine, Nuclear Submarine
This special class of modern naval units has a unique ability: to remain invisible to other units until it attacks, or unless it happens to finish its turn right next to the enemy ship. Also, its torpedo attack has a range of 2 (3 for the more advanced Nuclear Submarines) and always receives a 100% attack bonus (which grows further with promotions), which makes submarines one-shot killers! If that weren't enough, submarines are capable of entering ice tiles, which allows them to lurk where no other ship can, or escape pursuit. They are able to sneak up on a lone ship and sink it, then vanish again without a trace!
Submarines are weak in defense, however, and whenever discovered might die quickly, especially if attacked by specialized anti-submarine ships like the Destroyer and the Missile Cruiser. These two, along with other submarines, are capable of revealing them on the map so that all other ships can see and shoot at them. Finally, submarines can't attack cities.
To make the most of this terrifying unit, search for vulnerable unsupported ships with it, sink them, then retreat. Keep away from armadas, and never use your full move to a point on the map in the fog of war - this is a good way to end up right next to a ship you didn't see before, with no possibility of retreat! Also, thanks to their invisibility, submarines make excellent scouts in the late game.
Embarked units[]
All land units that move on water tiles via the Embarkation ability belong to this class; that is to say, your entire land army, while moving through the sea. Embarked units cannot attack (unless they're attacking a land tile, and even in this case they suffer a combat penalty), and they also suffer a defensive penalty which makes them vulnerable to both naval melee attacks and ranged attacks from sea and land. The only exception to this are the Marines, who are specially trained for sea combat - they don't suffer a penalty when defending on the water or when attacking from it; even so, they can only attack cities and land units. Also, the Songhai civilization is a specialist in Embarkation, and all their Embarked units (even as far back as the Classical Era) receive the same promotion as the Marine, and can hold their own when attacked.
Embarked units move very slowly, compared to ships - only 2 MP. Later, technological research increases their speed to 3 and finally 4 MP, but they're still quite a bit slower than the ships of their era, so be very careful! Also note that embarked units have reduced sight radius - they can't easily spot approaching enemy ships!
Always escort your embarked units with a fleet, unless you want to lose them easily! Many an invasion has failed because the invader had chosen to move his or her army through the sea at the wrong moment, or hadn't accounted for the naval strength of their enemy.
Air units[]
Air units are a late game feature which completely changes the modern combat tactics. Their ability to stay out of harm's way in cities makes them nearly invulnerable, if used right; their great striking range (6+ tiles) allows them to hit an invading army even before it approaches, or to attack targets which think themselves safe deep in enemy territory. At the same time, they always suffer a retaliation attack, much like melee units. So, we can say that they're a combination of melee and ranged unit, capable of striking from great distance but suffering damage in the process.
For a detailed description of the rules of air combat, check here.
Fighter aircraft[]
Types: Triplane ( only), Fighter, Jet Fighter
These airplanes are meant to defend against other aircraft, and to eliminate enemy interception abilities (both land- or naval-based and airborne). They can be specialized in either task via promotions, and become quite good at it. Fighter airplanes are very dangerous against other air units, but if you want to use their interception ability, remember not to do any action in this turn! If you do something, this unit will lose its interception attack for the turn. So, think carefully about which units you'll use as defense (by setting them on Interception mode and not doing anything else with them), and which ones you'll use offensively, doing air sweeps to clean up the way for bombing runs.
When you base fighters on Carriers, they will effectively defend your fleet from air power.
Also note that all fighter airplanes have the passive Air Recon ability, which reveals the fog of war in a great range around their base on the next turn. Use this to plan your actions!
Bomber aircraft[]
Types: Great War Bomber ( only), Bomber, Stealth Bomber
These airplanes are specialized in dealing damage, either to land, to naval units, or to cities. They are quite vulnerable to anti-air capabilities, so only use them after you've dealt with enemy interceptors, or that there aren't any in the area.
The promotions for Bomber units allow specialization on the types of targets, as well as uber-abilities such as Air Repair (unit Heals while acting on the same turn) or double attack. The last unit in the class, the Stealth Bomber, enjoys an additional defensive bonus vs interceptors, but it can't base on Carriers. Usually its huge Rebase range more than makes up for that, however.
Helicopters[]
This class is represented by only one unit: the Helicopter Gunship. This unit is quite unique, because it is a hybrid land-air unit: it can stay out in the open and move around the map as a normal land unit and it isn't affected by Interception, but at the same time it's vulnerable to anti-air attacks, so a Mobile SAM, for example, could attack it and eliminate it easily. Use the helicopter in hit-and-run attacks as you would a cavalry/armored unit - its specialty, in fact, makes it the best anti-armor unit in the game.
Anti-air units[]
Types: Anti-Aircraft Gun, Mobile SAM, Destroyer, Missile Cruiser, Triplane ( only), Fighter, Jet Fighter
The anti-air unit class is a broad one that includes land, naval, and air units. All of them share a common ability: Interception, which is specifically designed to counter air raids. Some of them are better at it than others (fighter airplanes being the champions), but all of them also share a Combat bonus vs Aircraft, which makes them terrifyingly effective. There isn't much strategy involved here - just move these units along with your main army, positioning them on the route of possible enemy air raids and keeping them idle so that they can use their Interception ability.
Always have some anti-air units with your army/navy, if you don't want to see them gutted by enemy bombers!
Healing[]
Healing is the process when a damaged unit (one that is bellow its maximum health) recovers health points. For the rough details on that, check here. But know that healing is of the utmost importance for a commander! You simply cannot afford to just throw away your units by letting them battle to the death all the time - units have to be produced in cities (which also takes time and bars the city from producing buildings), and new units may not have the promotions older, more experienced units had. So, you must learn how and when to let your army recover their health.
The basic thing you need to know is that under normal circumstances a unit may start healing whenever it's not doing anything else in that turn. Land units may heal everywhere, while Naval units may only heal while inside friendly territory. You can even heal in the midst of a battle by using the Fortify command. There's a special promotion - Medic - which speeds up healing; acquire it with at least one or two units of your army, and use them as battlefield healers.
In a battle, try not to lose units by pulling them out of danger once their hit points fall too low, or by Fortifying them. Don't keep attacking with a unit until it drops dead! In big battles, try to have a part of your army constantly healing while the rest engages the enemy. Keep an eye on enemy ranged units - they may focus fire on one unit, destroying it pretty quickly. After every major battle, don't move your units away immediately! Instead, spend some turns healing your army, so that they're ready to enter combat again with fresh strength. Remember that units always heal the fastest in cities.
Combat bonuses[]
These are additional factors which increase or decrease the CS of the unit, affecting its final performance on the battlefield. You can always see the applicable bonuses in the combat preview popup which appears right before you confirm an attack order - positive bonuses appear in green, negative in red; each on the appropriate side of the table. There are several classes of bonuses:
- Terrain bonuses - These depend on the terrain where the battle is fought (NOT necessarily the one a unit is occupying currently). They may be automatic (active simply because the unit is on certain terrain), or acquired via promotions.
- Formation bonuses - These depend on the positioning of units related to other units. They also activate automatically; the special 'Discipline' formation bonus requires adopting the relevant Social policy, though.
- Special bonuses - A number of other bonuses may apply, for example those that represent special abilities of units, or the combat aura of a Great General/Admiral.
- Fortification bonus - This bonus is acquired when you order a unit to Fortify. The unit digs in, creating a makeshift defensive position on the spot. This gives him a 40% defensive bonus, but it also makes it unable to attack or do anything else but Heal. Note that a unit may break Fortification mode at all times without any conditions. You can thus use the command to create temporary fortified positions on the battlefield, barring enemy advance, or to limit the damage a unit is receiving, while also giving it a chance to recover its health.
You will need to learn all major bonuses that apply and aim at taking advantage of them during combat - this is an essential point of the successful commander!
Combat domain analysis[]
When entering combat you need to be aware of where that combat is going to take place, because in Civilization V this will also affect all battles. There are two main domains as far as combat is concerned: land and sea. Air power acts in both domains equally - that is, its performance isn't affected by where the action is taking place.
Land combat[]
This is where the majority of combat takes place, not because there's more land than water - in fact, the opposite is true - but simply because cities and most of the important resources are found on land.
The main feature which affects land battles from a tactical view is terrain and how various of its features act as obstacles for movement and provide cover for many land units during battle. The latter, known as "defensive terrain bonuses," always works for the unit which occupies the relevant tile; the former works for all units on the field (unless they fly).
Hills, forests, and jungles provide a 25% Combat Defense bonus. Look for such terrain when placing your army to provide additional defense for your units. At the same time, these same terrains act as obstacles for ranged units, barring their view - an Archer, for example, won't be able to "see" the tile immediately beyond a forest, and as a consequence, it won't be able to attack it! This limitation doesn't apply to units with the Indirect Fire ability, as long as another unit provides vision of the relevant tile. Note that a city ignores sight barriers - it's able to shoot at all tiles in its range of 2, regardless of their features; a city itself also doesn't constitute an obstacle for ranged units. Bearing this in mind, you can use terrain features to limit enemy archers' effectiveness, but try to move your own archers in such ways as to circumvent terrain obstacles.
Marshes and flood plains provide a 10% Combat Defense penalty. This means that units occupying them and forced into battle there will not fight at full strength! Always be wary when entering a vast marshland or when fighting in deserts near rivers - the enemy occupying this territory could make your life a living hell. Because these terrain types are open (they don't obstruct line-of-sight for archers), archers positioned nearby will be able to shoot freely, while your units will be bogged down trying to traverse the difficult terrain, and this is a sure recipe for disaster. So, try to maneuver in such way as to avoid placing your army in the difficult terrain...or better yet, try to lure your enemy there to finish them off.
Another important point is that you (or your enemy) may construct defensive fortifications on land tiles - Forts or even Citadels. Units occupying these fortifications receive a large defensive bonus, which also stacks with terrain bonuses. Look to construct such defenses in important choke points on the battlefield, then place ranged units there. Note, however, that an enemy can also use your Forts (but not your Citadels).
Finally, land units may specialize in fighting in certain terrain types via promotions. This will give them a combat bonus in the appropriate terrain, but this time the bonus applies not only when they defend, but also when they attack. Again, this bonus stacks with any other natural or fortification bonuses. Also, we have Formation bonuses to consider, but they will be discussed later.
The second major consequence of terrain is movement impairment. All the above-mentioned terrains, excluding flood plains, require double movement to traverse. Since there are large expanses of forests and jungles on the map, land units will have a very difficult time moving there. Rivers consume all remaining MP for a traversing unit, which requires quite some planning on the side of a general positioning his army! Mountains can't be traversed at all (except by air units and Carthaginian units owned by players who have generated a Great General), and act as natural barriers to invasions. All of this is very important in the early game and greatly restricts the pace of combat - it is simply impossible to reach your target fast, or before they notice your forces!
All this changes with technological advancement and the construction of Roads and later Railroads. These allow a land unit to move across the map much faster, and negate the river crossing penalty; however, note that roads only work for a country which allows their use, not for an invading army! So, for example, when you're defending against an invasion, you will be able to make full use of your road network to move units between cities very fast, while the invading army will have to use the natural terrain, as if no roads existed! This gives great tactical advantage to the defending party, although if the attacker is strong enough, or positioned well enough, this won't matter.
Sea combat[]
Compared to land combat, sea combat is relatively straightforward. There are no terrain bonuses here, and all tiles cost 1 MP to move through. However, in the whole first half of the game, before the Renaissance Era, naval units may only move on coastal tiles which greatly restricts their movement, and consequently their maneuvering during battles. It is often possible to simply block enemy movement to a certain region by placing two or three ships next to each other - the enemy will then have to fight his way through them. This restriction is eliminated in the later game, when ships become able to move across all Water tiles. At that point the ocean becomes one huge battlefield, in fact - too huge. It is so large that it's practically impossible to keep it under control at all times. So, consider dividing your navy in several small groups to patrol the seas which interest you - that way you'll never get nasty surprises like a sudden overseas invasion.
Formation bonuses also apply in sea combat, although there are far fewer occasions for that, since a good part of sea combat is ranged, not melee.
The most interesting part of sea combat is when it actually crosses with land combat - when ranged units from both domains bombard each other. Never forget that your navy is susceptible to that, as well as your land units, and plan accordingly! Also, don't forget that a coastal city can harbor a naval unit in its tile - that unit comes as a "bonus" defense for the city (in addition to any land unit garrisoned there), also being invulnerable and able to shoot freely. However, that unit will be lost along with all others if the city is captured.
Don't forget that when land units embark or disembark, they lose all remaining MP for that turn! Also, don't forget that while embarked, land units are quite vulnerable!
Early game strategy[]
Barbarian skirmishes[]
These will be your first battles, so they deserve some analysis. Barbarians are normally unorganized (that is, they don't have armies, but separate units rampaging here and there), and you will have to deal with them mostly in the early game, when you yourself don't have a strong army. Initially Barbarians will only have Brutes and the occasional Archer. Thanks to the bonus vs Barbarians you can usually deal with those with your scouting Warriors, and later your own Archers will provide you a considerable advantage. So, don't stop your scouting when encountering loose Barbarian units, nor rush back to defend your city! They will be able to hold their own, especially if you are using terrain wisely.
However, attacking Barbarian encampments with only a single Warrior may be risky, because of the Fortified unit inside - it will resist many attacks, while continuing to spawn Barbarians who will wear down your solitary Warrior. For this reason, it's preferable to take on encampments with two or more units, and always heal your units after skirmishes!
Around turn 50 the Barbarians acquire a new and rather dangerous unit: the Hand-Axe. This is a ranged unit with a Range of 1 which is considerably stronger than a Warrior and can take quite some beating while also being able to attack without retaliation! This unit is fully able to defeat a solitary Warrior, unless he is fighting on familiar terrain and has some defense against ranged attacks. Also, around this point a critical mass of Barbarians may form if you've failed to clear up the first encampments, and you may find yourself swarmed by Barbarian units who will also become quite bold. They will gang up on your units, performing flanking attacks, and generally being quite aggressive. From this point on you'll have to take Barbarians more seriously, because a little later they'll also start producing Spearmen, Horsemen, and other advanced units and that will make them much more versatile tactically. Instead of sending solitary units after them, organize a task force of melee and ranged units and embark on a quest to clear encampments. Also, always check what unit is inside the a Barbarian encampment - an Archer can be taken down easily by any melee unit, despite being Fortified, while a Spearman means trouble.
Around turn 150 you should have gathered a sizable army, cleared most encampments, and started focusing on a more important task: fighting your rival civilizations.
Land battles[]
Early battles in Civilization V (through the Industrial Era) are characterized by tight formations, where positioning is extremely important. This is because of the limited mobility of most units (having only 2 MP in terrain littered with obstacles), and of the maximum range of ranged units - 2 tiles. This means that armies need to get pretty close to each other to fight, at which point (quite realistically) the battle turns into a huge melee. It also means that Formation bonuses will be applicable (almost) all the time, so you should group your troops well! In the absence of area-of-effect weapons like nukes, you can only benefit from grouping. Finally, try to acquire and always use a Great General in battles - the tight formations are ideal for maximizing his beneficial effect.
The basic military formation is simple: infantry units in the front; ranged/siege units behind them; anti-cavalry units on the sides close to the Archers; Mounted units - also on the sides, ready to circle and strike at the enemy line (this sort of formation you can find in any basic textbook about military strategy). Your archers rain death on the enemy they can reach (Siege machines too, although they're less effective at that), weakening it along with cavalry. Then your front line engages, advancing. At all times look for weak links in the enemy line such as stray units, and undefended archers, and target those first.
When the main melee starts (both front lines engage each other), Formation bonuses become extremely important. Try to always attack in such a way that another unit of yours is flanking the target; at the same time avoid attacking when an enemy unit is flanking your attacking unit - this will result in a negative bonus! Use your cavalry as an auxiliary to flank the enemy: move it right next to a target unit next to your own melee unit; attack, but don't retreat before also attacking with the melee unit; then retreat with the cavalry. In this way, you'll have used flanking bonus for both attacking units and moved the cavalry out of harm's way. Consider Fortifying parts of your line while inflicting damage with other parts or ranged units.
Be very careful when breaking the enemy line: when your melee unit destroys an enemy melee unit, they move into the tile the enemy was occupying! This often results in your unit being surrounded and in range of half the enemy army, in which case it won't survive long. To avoid that, try finishing off enemy melee units with your archers - this will preserve your line.
Of course, such ideal situations rarely play out on the battlefield; consider the fact that movement around enemy lines is more difficult, throw in the terrain and you'll find out that some battles may turn into real tactical nightmares! That's why it's so important to consider the terrain.
Terrain considerations[]
Terrain plays a huge role in early battles, both as obstacle and opportunity. Obstacle because it limits the movement of troops, and opportunity because it offers them defensive bonuses, blocks enemy archers' line-of-sight or offers your own archers vantage points for shooting. Get acquainted with terrain on potential battlefields and think ahead of time of good features you could use in a battle! Hills are vantage points to place ranged units on, surrounding them with defensive units like Pikemen. Forests and other terrain obstacles have to be considered carefully, because they could offer the enemy advantage against your advance. One very dangerous situation is several enemy archers well-placed on hills, and surrounded by difficult terrain. In this situation they can shoot freely on you (because of their vantage point), but you can't respond. Your melee units will also have a difficult time moving through the terrain to reach the archers' positions - they may find themselves dead or severely weakened when they finally manage it! In such situations you should either use faster troops (cavalry), or try to move units from all sides at the same time - this way the enemy may spread their fire, dispersing their damage output among all your melee units and allowing them to approach relatively unscathed. Of course, you may use these tactics yourself for defense.
Another important point, especially in the very early game, is that terrain will often slow down the movement of your own troops between your own cities. Before you establish your Road network, and even after that, you should be prepared to lose several turns to move units from city to city (either because the roads cross rivers, because the cities are simply too far, or because the roads themselves aren't finished yet). If there is danger of an enemy invasion, try to keep a scouting unit in terrain where that invasion would come from (that is, in the direction of enemy lands), so that you can receive early warning of an advancing army. This way you'll have enough time to move troops to the endangered city.
Finally, terrain is important because most of the combat-oriented promotions are terrain-oriented. When choosing a promotion, consider where is most likely your units will have to fight - rough or open terrain? If your early cities are in jungles or hilly terrain, and you have no intention of invading others, then concentrate on rough terrain promotions; otherwise, choose open terrain. Conversely, if you intend to start an offensive, look at the prevailing terrain close to your target, and concentrate on the appropriate promotions. Well-chosen specializations may mean the difference between winning and losing.
Reconnaissance in the early game may become a real pain in the posterior because of the limited mobility/visibility of your units. For every player that is thinking more strategically, it might be worth it to invest in a couple of Scouts with their Sight promotion - they will be able to keep an eye on large portions of land and notice the forces of the enemy inside their territory when you're preparing an invasion.
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In the early game naval battles are limited by the constraints of the coast, and also by the fact that (almost) all early naval units are melee. The first ranged naval unit only appears in the Medieval Era. That means that Formations again play a huge role in naval battles, although maneuvering is by necessity very limited. Melee ships have two promotion lines: one anti-naval and one for cities. Usually the first line is preferable, because attacking cities with ships can't be your main method of taking them anyway; only choose these promotions if your target has mostly coastal cities and you have a really strong navy. Note that the Coastal Raider promotions also allow your ships to loot 
Gold from cities - it's quite risky, but you can use this as an additional way of earning income throughout the game!
The addition of ranged ships in the Medieval Era starts making naval battles more interesting, because these ships can not only bombard ships from 2 tiles apart, but can also attack land tiles, bombarding units and supporting military action on land. Consider developing your navy at that point, especially if you're a sea-faring nation with the relevant bonuses. A navy will become very useful from the Renaissance Era on for such nations. Unlike melee ships, ranged ships can choose promotions specializing them either in naval or land bombarding - in most cases the second line is more useful, as it allows your ranged ships to rain death on land.
Another important change in the late Renaissance comes with the invention of the Privateer, a strong melee ship capable of capturing enemy ships. This makes naval combat considerably more interesting, as it is now possible to win battles even at a disadvantage if you employ your forces right. Use your Frigates' focus-fire and weaken an enemy ship, then your Privateers to deliver the coup-de-grace for a chance to convert the target ship to your fleet! At the very least a converted ship will present another target for enemies, drawing fire away from your main fleet. The less advanced a ship is, the better the chances of capturing it, so this strategy works best against Renaissance Era units and becomes less effective in subsequent eras, until the time comes to upgrade your Privateers to Destroyers, which carry on the promotion and bring up the conversion chance again. Privateers are good as front-line ships, although Frigates also work for that. Caravels are usually pretty weak in major naval battles, keep them as scouts.
Privateers also start with the Coastal Raider promotion, making them good city-harassers from the start. You can annoy the enemy by pillaging sea improvements with the Privateer, then attacking a coastal city, then escaping before he or she can react - which is not too difficult thanks to the Privateer's 5 MP.
Sieges[]
In the early stages of the game a siege can become a very messy affair, especially if the city is well-situated, in difficult terrain or on a hill. At this stage all your units, whether melee or ranged, will have to enter the defensive perimeter of the city to attack (because none has a bigger range than 2). The city ranged attack will deal heavy damage to attacking units, especially siege engines - never move a solitary siege engine within range, because you'll lose it right away! Instead, move several melee units around the city, then Fortify them, and only then move siege units in range for attack. Because of the set-up requirement, they won't even be able to attack in the same turn - they'll have to lose their remaining MP to set up, then attack only on the next turn.
Your main damage-dealers in a siege are the siege units and other ranged units; melee units get horribly damaged if they attack (because the CS balance is almost always heavily in favor of the city), and cavalry is simply useless against cities. That's why it's a good idea to Fortify melee units after they get around the city - their purpose is to draw its fire and survive as long as possible, while the ranged/siege units reduce its defenses to rubble. Now, carefully examine the terrain around the city - early siege units can't shoot over obstacles, so you might find they won't be able to reach the target if there are obstacles between their position and the target! Think about that when closing in the siege, and leave appropriate positions for your siege units, with clear line of sight to the city, or on vantage points! And when there are little to no appropriate positions (when the city is surrounded by rough terrain on all sides), know that you're in for a really difficult siege - prepare a strong army for it!
The city AI tends to attack damaged units first, so you may attract its attention by yourself attacking with a strong melee unit, then Fortifying it. The city will (hopefully) lose 2-3 turns firing at that unit, leaving your siege units to do their work. If you can, move hurt units away from the city to save them; otherwise, just accept the losses and continue pushing until you drop city health to about one quarter. At that point you may attack with everything you have - the city should fall. And remember that you always need to have at least one melee unit to finish off the city - otherwise you can't capture it!
Of course, sometimes the enemy also has an army defending the city (rather than only 1-3 units) - in this case you'll have much more work. Use your melee units to deal with the defenders, along with cavalry, if you have it, while concentrating your siege units on the city (they won't be that effective against the other units anyway). Of course, sometimes even that won't be enough - if the defending army is close to the size of your army, don't even bother attacking - you won't be able to take the city. The combination of its devastating attacks and the difficult movement around it will wipe out your forces before you manage to inflict enough damage, and even if you manage to take the city, the enemy units may retake it right away! In this case, retreat and try to thin out the defenders first.
Defending against a siege[]
In the early game a city is a terrifying addition to your army. It can shoot everything within 2-tile radius (without line-of-sight considerations), deals a lot of damage (both to melee attackers and with its own ranged attack), and what's more - its damage doesn't diminish when itself sustains damage. Even better - it will serve as a secure base for one unit, and offer your units free movement through its tile, while hindering enemy movement. You should use all of this to your advantage!
Evaluate your neighbors, and the danger of their invading. If you judge it high, then you should fortify you frontier cities, building Walls and later Castles, and leaving economic development for later. Do not clear the surrounding terrain of vegetation, if you can help it - this will slow down the attackers considerably, while providing your own defensive troops with cover and thus making them more resistant.
Always keep a ranged unit as garrison (it will deal more damage to attackers than a siege unit); in a coastal city, put a ranged ship in it too to increase your damage output even more. In general, ranged units are of much more use when defending a city than melee ones.
Pay attention to the size and quality of the attacking force: if it isn't too big, then attack siege units first to limit the damage your city sustains, leaving melee units to exhaust themselves attacking the city. On the other hand, if the invaders are very strong, then try to destroy melee units first. The logic is that if the enemy has no melee units, they can't take the city, even if its health drops to 1. This way, you can finish off the remaining ranged units one by one.
The enemy AI usually attempts to surround the city, or at least to move as many of its units in range as it can before starting the real attack. This leaves you some rounds of free hits, before the invaders start damaging you in earnest. In most cases it's better to draw your defending units behind the city, leaving it to act as a bulwark against the enemy. If the city is on a river, form a line on the far side of the river - this will make it very difficult for the enemy to move and surround your city. Its melee units will kill themselves attacking the city, or trying to cross the river, while you can easily pick out the ranged attackers one by one. The only case when this is not recommended is when the enemy possesses lots of ranged/siege units, with which it could focus-fire your city from one side, and drop its health quickly. In this case you should concentrate on eliminating as many of them as early as possible, and then retreat behind your city.
Late game strategy[]
The late game introduces to combat far greater mobility - both because of more sophisticated transport methods, and because terrain features acting as obstacles, like forests and jungles, have been gradually removed. When you finish the Railroad network of your empire, you'll be able to move entire armies across continents in 3 or 4 turns; and when you equip cities with Airports you can move armies in 1 turn only! Of course, you should be aware that the enemy will also be able to do that - it won't matter that your reconnaissance shows little defense around a border city, if the enemy may move half his army there in the next turn! Again, the defending party will have an advantage in this regard, but keep in mind that the attacker's mobility is greater than in the early game, because of less obstacles and/or faster units.
But the major change in combat strategy that the late game introduces is air power. Airplanes act a lot like a super-ranged weapon, able to strike at units many tiles away from their base, ignoring terrain - and since they are not actually on the battlefield, they can attack without being exposed to deadly counterattacks! At the same time, airplanes suffer retaliatory damage like melee units, which means they aren't a super-weapon. Still, the relative damage they suffer is not serious, and the capability of concentrating air attacks on a single enemy unit more than makes up for the risk, especially if the attacking airplanes are specialized via promotions. For example, you may focus-fire on a single unit, and destroy it within the space of 1-2 turns with air power, no matter its strength! After that, the airplanes may take 1-2 turns to recover health in their base, without fearing death! This makes airplanes a very important tactical weapon, able to breach enemy lines with ease. It also means that defending against airplanes becomes of utmost importance on the battlefield.
For the concrete mechanics of Interception, the main way of defense against air power, check here. But to make a long story short, you need specialized anti-air units with both your land armies and navies to defend against air raids. You can also use air-based defenses (i.e., fighter planes). Each Interception-capable unit will defend against a single air attack per turn, inflicting heavy damage on the attacking plane, and possibly even destroying it before it can reach its target! Remember, however, that all anti-air units have a range in which they can Intercept - if they're too far from the tile the airplane is attacking, they won't be able to use their Interception! So, from their invention on you should produce anti-air units and move them with your armies at all times.
Attacking with air power[]
When attacking with airplanes your main weapon is the bomber unit. Move as many as you can in a base as close to the target city/battlefield as possible. Always move also at least one fighter unit for the valuable Air Recon ability it provides, which will allow you to see everything nearby. Now, if you see a lot of anti-air, you should bring more fighters and initiate each turn with Air Sweeps around the target you wish to attack - this will eliminate Interception danger. After that, sweep with your bombers. Always keep an eye on their health - don't let them drop too low on health, or an unexpected Interception may destroy them!
Note that certain land/naval targets are more dangerous to attack than others - anti-air units receive a considerable bonus when defending against air attacks! It also seems that ranged units have a hidden bonus against aircraft, because they are more effective in their retaliatory damage than infantry or tanks. The best targets (as usual) are siege weapons, which get devastated by air raids.
Note that your land bases (cities) are immovable, and in rare occasions you'll find that your air power can't reach certain areas. In such cases, try to use Carriers to aid your attack, positioning them in such a way as to reach the area in question. And, every time you initiate an overseas assault you'll need Carriers to transport your air power - defend these at any cost!
Defending against air power[]
When defending against air raids, your main weapon is fighter units, and of course all land- and naval-based specialized anti-air units. When in danger of raids, move as many fighters as possible in a nearby base - their Interceptions are the best, and they alone have a promotion allowing them 2 interceptions per turn. Don't attack with fighters when you need to defend against air assaults! This will waste the Interception ability for this turn. Instead, leave them in Interception mode.
As for the rest of your anti-air capabilities, try to position them on the battlefield close to eventual targets of enemy air raids. Note that Interception has a maximum range - if the attack occurs outside that range, your interceptor won't be able to react!
Land battles[]
Besides air power, modern times also bring to land battles increased mobility (armored units) and other units capable of striking from a great distance - the siege units. At the same time, front-line units retain their basic qualities, cavalry transfers straight to tanks and the old ranged units now upgrade to basically front-line units (since they can't attack 2 tiles away anymore, but on the other hand their CS now equals their RS). So, form a front line of mixed infantry and ranged units, which will support each other - the ranged units will inflict damage on the enemy front line, while the infantry will play a more defensive role.
Anti-air units also are basically melee fighters, although you shouldn't keep them in the front-line - they're much more valuable as anti-air assets than as melee units. Still, they should be as close as possible to the probable targets of enemy air raids, such as your siege or tank units. As usual, siege units go at the back, their increased range now being capable of reaching targets further away (and also thanks to the Indirect Fire ability, they don't necessarily need vantage points anymore), and your tanks (the contemporary cavalry) will stay on the sides, ready to strike at vulnerable parts of the enemy line, then retreat again.
When attacking enemy positions you can now use air power to reach targets beyond the enemy lines; you can also use a special new unit - the Paratrooper (and also the XCOM Squad in Brave New World), which is a decent infantry unit capable of infiltrating behind enemy lines to attack vulnerable targets. But note that you can't rely on these weapons only to defeat an entire enemy army! You will need your own full army with infantry and armored units at least.
In defense, bear in mind that your front lines don't provide such a sure defense anymore. The enemy may also use the above methods to breach them and attack vulnerable targets from behind. So, take care and don't put all your strong units in the front, unless you've noticed that the enemy doesn't have much Air power, but has serious infantry/armored/ranged divisions! In such a case, the battle may deteriorate into an old-fashioned melee - use the old tactics to defend.
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At sea the scene also changes, and again we have much more mobility (thanks to all types of ships having 5+ MP per turn). This means that fleets can now cover much more sea than before and appear literally out of nowhere to attack you, which makes naval battles much more dangerous and unpredictable. The greatest change here comes with the Submarine class, capable of staying invisible most of the time. Submarines have a normal sight range and decent speed, making them great as late game scouts, but they're much more dangerous as sneak attackers! As mentioned above, in most cases Submarines can severely damage targeted ships, and sometimes they can sink them with a single shot! Given the immensity of the sea, and the fact that submarines can also hide in ice tiles, it is incredibly difficult to notice them in time; this makes solitary targets incredibly vulnerable to submarine attacks (unless they're specialized in submarine combat, like Destroyers and Missile Cruisers). So, don't move solitary ships or embarked units around unknown seas! And start doing regular sweeps of important seas with anti-submarine units. Don't use your full move, because you may miss the enemy! Instead, advance a couple of tiles at a time, and if you notice a submarine you'll have the chance to approach and attack it in your remaining move.
From this point on the concept of a navy becomes really important, as important as a land army in formation.
A typical navy formation puts vulnerable ships like Carriers and embarked units in the middle, Battleships and Missile Cruisers in a tight formation around them, and Destroyers and Submarines circling the whole formation, back and forth, making sure no enemy submarine or aircraft can sneak in and deliver, say, a Nuclear Missile. Great Admirals also become valuable at this point, because their bonus can now benefit lots of units at the same time.
Modern Ranged ships have a Range of 3, which allows them to attack from a great distance - use this to your advantage when engaging enemy formations. Keep your Destroyers as a front-line defense, while pounding the enemy with Battleships, Missile Cruisers, Submarines, and air units. When you manage to breach their formation, move in with Destroyers to attack vital targets such as Carriers, or the unit hiding their Great Admiral - losing a unit or two is worth it if you sink these. Note that Carriers become priority targets in the sea because of the air power they carry, which can completely alter the balance of power. Remember, your naval-based defense against aircraft are Destroyers and Missile Cruisers - use them the same way you would land-based anti-air.
And of course, greater range means greater ability to attack forces on land. Coupled with the Indirect Fire ability, Battleships now turn into a major threat to land-based armies, capable of large-scale support. When assaulting from the sea, thin the enemy army defending the shore first, then Disembark your units and continue the push. Marines are a great asset in such situations because they are capable of attacking tiles on land without disembarking first, and without penalty (all other embarked units suffer a penalty). So, when you plan an overseas offensive, build some Marines and use them as the first line of attack on the shore, while shooting with your Battleships. And of course, don't forget to bring your own air power with some Carriers!
Sieges[]
In modern times, sieges become both easier and trickier. It's not about brute force or maneuvering anymore, because even an overwhelming army can fail to take a properly defended city. The greater range of some key weapons allows you to shoot at cities from safe distance, unlike in the early game. This means that in the ideal situation you can break the enemy defenses with siege weapons outside of the city defensive perimeter, then sweep in with a single armored unit (which, unlike mounted units, suffer no penalties when attacking cities) and take the city without any losses whatsoever! Of course, don't forget that you need to see the city in order to use the Indirect Fire ability of siege units - sometimes the city is so situated that you will need a unit right next to it for visibility.
If the city has an army defending it, move your infantry into its defensive perimeter and engage the defending army with them to keep it away from your Siege units in the back.
Of course, Air power here matters again. Because it's so natural to have it in cities, and because bombers are so terribly effective against siege weapons, you might find your besieging army quickly decimated by a few enemy bombers. So, you'll need to bring anti-air to counter that; keep these units close to your siege units, and outside the city's defensive perimeter. And in the converse situation - when you're defending a city against a siege - use your Air power to deal with ranged units outside the city defensive perimeter.
Nuclear war[]
There is one more weapon that can make a huge difference in modern combat: nukes.
Nuclear weapons are the only area-of-effect weapon in the game. Their production requires 
Uranium and quite a bit of tech research, which proves difficult and time-consuming; also, each nuke is a one-time use weapon. However, they are truly devastating, especially against field forces, as they are capable of obliterating most units of the time and very seriously hurting the others. Any civilization with nukes that decides to use them will radically alter the situation on the battlefield.
Not much can be done to defend against the two types of nukes, especially if they are launched from cities. The Atomic Bomb is in fact a bomber unit, so it is supposed to be possible to shoot it down. Unfortunately, due to a bug that has been present since the game's launch, it is impossible to intercept the planes that deliver the Atomic Bomb. The Nuclear Missile is similarly impossible to stop once launched. However, Atomic Bombs can be housed on Carriers, and Nuclear Missiles can be housed may be housed on Missile Cruisers and Nuclear Submarines, so try to target these units in hopes of destroying them before they have the chance to reach their target.
In addition to the damage they do to military units, nuclear weapons reduce the 
Population of cities in their blast radius, decreasing their productivity and making them easier for infantry units to capture before they can muster a strong defense. Moreover, Nuclear Missiles will utterly destroy cities with a low enough Population, so "scorched earth" tactics are easier than ever in modern times.
Finally, be warned that using nukes will result in a hefty diplomatic penalty...especially with the civilization on the receiving end of the nuke! It is highly inadvisable to use them if you're not headed for a domination victory, when you won't be able to escape hostile diplomatic relations anyway.