War, what is it good for?
In Crusader Kings II, hopefully you'll gain some titles and in the best of worlds, imprison or behead your enemy. In order to declare a war you'll first need a valid Casus Belli against your enemy(a CB held by a vassal or courtier will of course do as well). You cannot attack the vassals of someone, so if you want one of their titles you have to attack their liege. Also, unlike Europa Universalis III and Victoria II, once a war has started it cannot be extended by adding further wargoals or CBs.
Instead, each CB has three options scripted: Demand Defeat, White Peace and Reversed Demands. As an example, let's look at the Claim CB. This CB lets you attack people holding titles you have claims on. If the war is going well, you can demand that your enemy give his title to you and as a bonus you'll gain a small amount of prestige. If you fail to achieve your goal, you could sue for a white peace instead. You'd want to avoid this though, since signing a white peace gives you a prestige hit(you didn't achieve your goals, after all). White peace is still preferable to the reversed demand however, since if your enemy enforces this, you will both lose the claim and get a huge prestige hit.
While most wars will end in a peace treaty, this is not the only way they can end. Some CBs have effects that trigger when the leader of an alliance dies. An example of this is the Invasion CB, which is currently used by William against Harold. When the leader of the attacking alliance dies, the war immediately ends. Be careful when going to war with your old king...
Another part of wars is the warscore. Like our other games, you'll gain warscore by occupying enemy holdings(the capital is worth more, vassal holdings are worth less) and winning battles(in CK2, they are worth a lot of warscore). We've also added a warscore effect if the contested title is left with no controller change for some time. After three years(currently), warscore is slowly added to the person controlling the area. This means that it's now possible to win a war as a) a defender in a war by just defending your title or b) attacking someone, sieging down the title you want and then just stand still and defend those provinces. By the way, if you manage to capture and imprison the enemy leader(for example, in a battle), this automatically counts as 100% warscore. We've also removed all limits to warscore, so whoever reaches >=100% first by any combination of occupation, battles, controlling the correct territory and imprisonment automatically wins the war.
Last but not least we have tagged some CBs to be "hostile against others", for example the Invasion CB. The effect of this is that two parties contesting the same title will fight each other even if they are not at war. It might be better as William to wait a bit until Harald and Harold both have worn down their armies...
'Till next time!
Programmer, Paradox Interactive