LONG DISTANCE WARS
Wars over a long distance can be particularly hard to manage because on the turn that you declare war you might be confident that you can win with your current units but if the units you have near they're cities are killed then you have a problem because by the time you have made some new units and got them back to your original position(After of having to deal with barbarian outposts or an enemy city state)anything could of happened for instance:
- They could have made enough units to destroy your new army and then go on to take your cities.
- They could have built or upgraded their walls so that they are beyond your ability to destroy.
- They could of pushed their troops forward so that your troops never even reach their cities.
All of the above could result in major setbacks or even loss of the game. So make sure you either have more than enough troops to take the other civ out the game or have some means of transporting your troops e.g sea
I recommend founding a city near the opposing civ so you can always have some means of reinforcement.