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Casus Belli is a new gameplay mechanic introduced in Civilization VI that allows you, after you meet certain prerequisites, to justify the wars you declare and get fewer warmonger penalties (Grievances Grievances in Gathering Storm). The alternative to using a Casus Belli is the Surprise War, which will give you a much larger warmonger/Grievances Grievances penalty than the Casus Belli, except in the Ancient Era in which there is no warmonger penalty. As the game goes on and you enter new eras, the penalties increase for both Surprise Wars and the various Casus Belli.

Most Casus Belli are unlocked by researching specific civics, as detailed below. As of the Spring 2018 Update, Casus Belli can be used when declaring a Joint War with another civilization. However, there are diplomatic situations in which Casus Belli cannot be used, such as when a leader protests your positioning of units on his/her borders, and you decide to answer by declaring war.

Types of Casus Belli[]

Surprise War[]

While technically not a Casus Belli (as they are defined as justifying a war and Surprise Wars are not justified in any way), this is the standard (and only) form of war for the early game. There are no requirements for declaring a Surprise War - you can do it immediately, whenever you please. Post-Ancient Era however, you will receive 50% greater warmonger penalties (150 Grievances Grievances) for such an outrage, which makes this the worst possible choice from a diplomatic standpoint. It is also worth noting that this is the only type of war that you can declare on city-states, since your interactions with them are very limited - you cannot denounce a city-state, for example.

Persian units under Cyrus gain Movement Movement bonuses for the first 10 turns of a Surprise War, and he incurs less warmonger penalties for declaring them. On the other hand, it is not possible to declare a Surprise War on Canada, nor can the Canadians ever declare Surprise Wars themselves.

Formal War[]

This is the first Casus Belli you can use to justify a war, and it's available right at the start of the game. A Formal War differs from Surprise War not because of any moral justification, but because it comes with a warning: it can only be declared after some diplomatic incidents (such as denouncing or being denounced by another leader at least 5 turns prior, or having a leader break a promise that he or she made to you), and as such comes as no surprise to any party. There are no restrictions on which cities you may capture. If it is the Classical Era or Medieval Era, this is the type of war you will want to use in order to avoid heavy warmonger penalties. The warmonger penalties for this type of war are standard (100 Grievances Grievances).

Holy War[]

This Casus Belli is unlocked with Diplomatic Service. It is used to declare war on a power that has religiously converted one of your cities, thus sparking religious outrage among your citizens. Given the importance of religion in this game, the other leaders tend to become more receptive to such a kind of war. All warmonger penalties are halved (50 Grievances Grievances), whether it be the declaration of war, capturing a city, or razing it. If you are playing for a Religious Victory and have competition trying to convert your land, this will be the type of war you are most commonly going to engage in.

Liberation War[]

This Casus Belli is unlocked with Diplomatic Service. It is used to declare war on a power that has captured a city from one of your friends or allies. Declaring war in such a case is seen as justified in any era; you will gain no warmonger penalties (0 Grievances Grievances) for the declaration of war itself, and so long as you restrict your conquering activities and return the captured cities to their original owners, no further penalties apply. This acts as a retaliatory war of some sort; if you have a strong ally that has been weakened, then you can use this to help them get back on their feet.

Scotland under Robert the Bruce gains major Production Production bonuses when declaring one of these wars, so look out for any possible opportunity to declare one. Try, however, not to actually succeed; if the enemy is still holding the city at war's end, you can declare another Liberation War shortly after and benefit from more increased Production Production.

Reconquest War[]

This Casus Belli is unlocked with Diplomatic Service (Defensive Tactics in the expansions). It is very similar to a Liberation War, but this time it relates to cities you founded - you can only declare this kind of war on the power that has occupied one or more of your cities, but you will incur no warmonger penalties (0 Grievances Grievances) for declaring the war and capturing such cities. This can also be used as a form of retaliation, or a way of justifying the restart of a war that ended previously. But remember, just because you're declaring a Reconquest War doesn't mean you have the right to capture any of the enemy's other cities, in which case warmongering penalties apply as usual.

Protectorate War[]

This Casus Belli is unlocked with Diplomatic Service (Defensive Tactics in the expansions). "Protectorates" in Civilization VI refer to allied city-states (i.e., those you are the Suzerain of), and with this Casus Belli, you can declare war on the power that has attacked them. Note that this Casus Belli remains active only while the power and the city-state are at war; if they make peace, or if the city-state gets conquered, the Casus Belli is voided, though an ongoing Protectorate War is not canceled if the city-state in question is conquered. As is the case with Liberation and Reconquest Wars, other leaders understand and admire this kind of loyalty, and there are no warmonger penalties (0 Grievances Grievances) for either declaring the war or assisting the city-state in any way. Again, you can use this if another force is becoming too powerful and is beginning to take down your neighbors.

Colonial War[]

This is one of the more modern Casus Belli, unlocked with Nationalism. It is used to declare war on a power that is two or more technology/civic eras behind you. Other leaders accept such a war as a way to "bring civilization" to an otherwise barbaric and backwards country. All warmonger penalties are halved (50 Grievances Grievances). This can be used effectively against empires that have been focusing on a different element, such as their military, and gives power to the more technologically advanced civilizations. When you're opting for a Domination Victory, you will find this Casus Belli useful for mopping up the weaker nations around the globe.

War of Territorial Expansion[]

This Casus Belli is unlocked with Mobilization. It is used to declare war on a power that borders your empire, and in whose lands you wish to expand. You must have at least two cities within 10 tiles of 2 of the opponent's cities for this Casus Belli to become available. Warmonger penalties are reduced by 25% (75 Grievances Grievances), since leaders frown upon expansionism, but can understand it somewhat. As the name suggests, this will be used to expand your empire when things start to get crowded, and can also be incredibly useful in domination games.

Chandragupta gains access to this Casus Belli with Military Training instead of Mobilization, and his units gain +2 Movement Movement and +5 Strength Combat Strength in the opening turns of one, which helps him expand the Indian empire across the globe.

Rise and Fall expansion[]

With the Rise and Fall expansion, some new Casus Belli have been introduced.

War of Retribution[]

This is one of the first Casus Belli in the game, unlocked with Early Empire. It is another kind of "revenge" war, which can be declared on a power that has broken a promise that was made to you within the last 30 turns. During a War of Retribution, you have a 50% reduction of standard warmonger penalties (50 Grievances Grievances) for declaring war and capturing cities, but razing cities gives you a 200% penalty. Although the enemy may have broken their promise to you, this does not entitle you to burn their cities to the ground in revenge.

Ideological War[]

This Casus Belli is unlocked with Ideology and represents the expanded role of this particular aspect in modern society, as in real life. It is used to declare war on a power with a different Tier 3 government: since all Tier 3 governments represent a different Ideology, other leaders see a (somewhat) noble purpose in your declaration and are able to condone it. This Casus Belli grants you a 50% reduction of standard warmonger penalties (50 Grievances Grievances) for declaring war, capturing cities and razing cities. The Ideological War is one of the most suitable for late game conquests, since its only prerequisite is for the target to have a different government, and this is very often the case. Of course, it will be impossible if either you or the target doesn't have a Tier 3 government!

Golden Age War[]

This Casus Belli is unlocked after you select the To Arms! Dedication when your empire has just entered a Golden Age Golden Age. There are no restrictions and all warmongering penalties are reduced by 75% (25 Grievances Grievances), which makes it hands down the best Casus Belli for the purpose of a Domination Victory when you have access to it.

The power of the Golden Age War is balanced out by the difficulty of earning and maintaining the Era Score to avoid a Dark Age Dark Age, let alone attain a Golden Age Golden Age. Furthermore, To Arms! is available only from the Industrial Era to the Information Era, and the Casus Belli itself will become unavailable once your empire's Golden Age Golden Age ends.

Related achievements[]

A Case for War
A Case for War
Declare a formal war.
The direct translation of casus belli.
Some Wine For Your Soldiers?
Some Wine For Your Soldiers?
Playing as Persia, conquer the original Scythian capital within 10 turns of declaring a surprise war on Scythia
During Cyrus' invasion of Scythia, he set up a trap where some of his soldiers set out a camp and drank wine. Scythians under Tomyris' son's command attacked the camp and drank all the wine there to celebrate. When Cyrus returned, he found little resistance in the drunk Scythians and slaughtered them.
The 25th Dynasty
The 25th Dynasty
Playing as Nubia, liberate the original Egyptian capital in a liberation war with the capital's conqueror.
The 25th dynasty of Egypt was in fact Nubian in origin after taking over from various foreign rulers in Egypt.
Claim the Fourth Cataract
Claim the Fourth Cataract
Playing as Egypt in a regular game, conquer the original Nubian capital within 10 turns of declaring a formal war on Nubia
Cataracts are physical barriers in at some points in the river Nile. The fourth one is where Nubia was generally situated.