Casus belli is a Latin expression meaning the justification for acts of war.
As with other games from Paradox Interactive, you need to have a CB to start a war in Crusader Kings II. You cannot declare war without a CB. There are several types of CB and spoils of war are dependent on what type of casus belli is pushed. You are only allowed to push a single casus belli in a war, so you should choose the one that benefits you the most. With the exception of the Tribal Invasion CB, all CBs incur a 10-year truce.
Most casus belli, if one discounts rebellion CBs, are based around claims. If you or one of your vassals have a claim upon a title in a realm, you can push this claim through war. If you have multiple personal claims upon a single realm you can push all of them at once, but you can only push a single claim for any other character per war.
If you have a claim upon a title you have a certain 'right' to this title. Upon winning a claim war you, or the person you were pushing the claim for, will gain this title, and if you were pushing it for someone their opinion of you will increase by 100, and you'll gain 100 prestige. If it ends in a stalemate you'll lose 100 prestige, and if it ends in loss, the claim will be lost in addition to 200 prestige. When pushing a vassal's claim and losing you'll have to pay concessions.
If you or a vassal hold a duchy, where part of this duchy is owned by another independent realm, you can push a de jure claim upon one of the counties held by the other realm. Even if the other realm holds more than one county you can only push a single de jure CB at once.
Unlike regular claims, de jure claims cannot be lost unless you lose control of the duchy. Losing a war merely means a loss of 200 prestige, while you'll still keep the de jure CB. If you win you'll gain the county as a vassal, or if you were pushing the de jure claim for someone else they'll gain the county as a vassal, have their opinion of you increased by 100, and you'll gain 100 prestige. When pushing a vassal's claim and losing you'll have to pay concessions.
There are several CBs based around changing realm laws, both crown and succession law. In total there are five, changing to Gavelkind, Elective, Seniority, or Primogeniture, and lowering Crown Authority.
The first four can be used on your liege as long as they don't have the succession type in question. The lower crown authority CB can be used on your liege if he's a king or emperor. You cannot use these CBs more than one step up the chain, so if for example you're a vassal of the King of Bohemia you cannot try to lower crown authority in the Holy Roman Empire. If you win the war, the law in question will be instituted. If you stalemate or lose you'll lose 100/200 prestige.
As long as you're someone's vassal, you can try to achieve independence through war. If you win you'll be independent and if the liege you're rebelling from is a king or emperor, crown authority will be lowered one step. If you stalemate you'll lose 100 prestige, and if you lose you'll be imprisoned and lose 200 prestige. Like with changing law, you cannot use this CB more than one step up the chain.
As long as no one else is trying to depose your liege, and your liege is not attempting to depose someone else or declaring independence, you can attempt to depose your liege. Upon victory crown authority will be lowered by one step (if the deposed liege is a king or emperor), and your liege will depose to the eligible heir you like the most. If you stalemate you'll lose 100 prestige, and if you lose you'll be imprisoned and lose 200 prestige. Like with changing law, you cannot use this CB more than one step up the chain.
As long as you're someone's vassal, you can try to overthrow him and take the title for yourself. Upon victory you'll gain the title and some prestige, and if the ruler you overthrew was a king or emperor, crown authority will go down one step. In addition, if your liege held any other titles, he'll be forced to abdicate them to his heir. If you stalemate you'll lose 100 prestige, and if you lose you'll lose 200 prestige and be imprisoned. Like with changing law, you cannot use this CB more than one step up the chain.
If you're a pretender to your liege's throne, or someone in your realm is a pretender to any throne, you can dispute the succession. Upon victory you (or the person you disputed the succession for) will gain the throne, and if it was a kingdom or empire crown authority will be reduced by one. White peace is impossible. If you lose, you'll be imprisoned, lose 200 prestige, and lose the claim.
If you have a claim upon a throne or you're smaller than the realm in question (that is, you have fewer holdings in your realm), and the Pope condones your invasion (costing you 500 piety), you can use the invasion CB. Upon victory you'll get the throne in question, 400 prestige, and every occupied holding. As such, you should occupy every holding before enforcing demands, as then you can install your own nobles. If you stalemate you'll lose 100 prestige. If you lose you'll lose 200 prestige, and have to pay concessions, and you'll lose your claim upon the throne.
You'll never get to use this CB unless you mod the game. It essentially just lets tribes like the Golden Horde and the Timurids attack anyone they neighbor, and take their lands. It works the same as the invasion CB for the most part. Any outcome incurs a mere 1-year truce.
If the Pope has called a crusade, you can declare war upon the target. Upon victory you will gain all holdings (as long as they're occupied by you, or unoccupied) in the targeted duchy and 100 piety. If you stalemate you'll lose 50 piety and 100 prestige, and if you lose you'll lose 150 piety and 200 prestige.
If you're in a different religious group than your target, your target is heretic, or you are heretic, you can declare a religious war. Upon victory you'll gain 50 prestige and all holdings (as long as they're occupied by you, or unoccupied) in the targeted duchy. If you stalemate you'll lose 50 piety and 100 prestige, and if you lose you'll lose 150 piety, 200 prestige, and be forced to pay concessions.
If someone has the same head of religion as you, and has been excommunicated, you can declare war upon them. Victory will give you 10 to 70 piety and 50 to 400 prestige depending on the excommunicated ruler's rank, and the target will be re-communicated and forced to abdicate. A white peace will lose you 50 piety and 100 prestige. Losing will lose you 100 piety, 200 prestige, and you'll be forced to pay concessions.
Added in the Sword of Islam expansion, the Conquest CB is used by Pagans and Muslims to attack any county directly bordering their own lands. It costs 50 Piety to use, and you must have the Piety for the option to show up. Victory gains you the county, or transfers vassalage of the county to you, and gives 100 Prestige. White Peace costs 100 Prestige. Losing costs 100 Prestige and forces you to pay concessions.
While some of these CBs you'll have merely due to your position, like the independence CB and the law change CBs, many of these can be gained on purpose.
First, the easiest type of CB to gain are claims. There are four primary ways to gain claims:
- Forge the claim. This doesn't works for anything above the ducal level (skill of 15 needed to forge a ducal claim)
- Marry someone who has an inheritable claim. Your children will have the claim and you can then push it
- Marry someone in your dynasty to someone who has an inheritable claim. Their children will have the claim, and you'll be able to push it
- Invite someone with a claim to your court. You'll then be able to push it
Second, de jure claims are also relatively easy to gain. If you can create a duchy that overlaps another realm's territory, this will gain you a de jure claim, and the same applies if you usurp it. In addition, after pushing a claim for a duchy, you'll often have de jure claims.
Third, dispute succession CBs can, like general claims, be gained by inviting someone with that CB to your realm.
Fourth, excommunication CBs are easy to get as long as the pope likes you, but these won't help you grow your realm. They're good for gaining some prestige, though.
Fifth, you can get religious war CBs on essentially the entire world if you convert to a Christian heresy. The easiest way to get heresy to appear so that you can convert is appointing an antipope, as this lowers the moral authority of the church. The lower the moral authority of the church, the more likely heresy is to spread. Then upon the first chance you get, you should change to whatever heresy arises. You'll now have CBs upon everyone except other realms following the same heresy. So while the drawbacks of heresy are severe (-30 opinion with all non-heretics, and increased rebel risk), the advantages when it comes to expansion are huge.
Using Your CB
Pushing your own claims and similar is rather straight forward. When pushing someone else's claim however you have to be careful, or they'll end up as an independent realm. They'll only be your vassal if they are already your vassal, they're in your dynasty, or the claim you pushed was for an area that is de jure part of your realm. If the title they took is at the same level as yours or above they'll be independent even if any of the above conditions apply, unless you have a liege above you where the conditions still apply.
The easiest way to ensure that they'll be part of your realm after pushing their claim is granting them a county in your realm first. The only way they can become independent then as a result of the claim pushed, is if the title they gain is at the same tier as yours, or higher.