Casus belli is a Latin expression meaning the justification for acts of war. In Crusader Kings II a valid casus belli (CB) is required to wage war against your enemies. There are many different types of CBs, and the CB used to declare war determines the potential outcomes at the conclusion of the war. Only one CB can be pushed per war.
With the exception of the Tribal Invasion CB, all CBs incur a 10-year truce. Truces only apply to specific characters - the instigator of the war and the defender. If either character dies the truce no longer applies. If the instigator of the war breaks the truce he or she takes a prestige hit, a -25 relations penalty from all characters of the defender's religion, and a -5 diplomacy penalty for 5 years.
In addition, Sunni Muslim rulers who are waging war with a fellow Sunni will lose 2 piety per month while the war is ongoing.
Types of Casus Belli[]
There are a number of different CBs. These are listed below,
Claim CB
Characters who do not hold a certain title, but have some sort of connection to it can hold a claim. Characters can use this claim as a CB to go to war to seize that title. Claim can be either strong or weak. Strong claims are given to those second or third in the line of succession. Weak claims are given to those lower down in the line. Strong claims can also be fabricated for a character by their chancellor.
Strong claims can always be pressed. Weak claims can only be pressed against:
- Female rulers if the claimant is male
- Regencies
- Titles already contested in a Claim or Succession war
- Titles the claimant is second or third in line to inherit
If you or a vassal hold a duchy or county, where part of this duchy/county is owned by another independent realm, you can push a de jure claim upon one of the counties/holdings held by the other realm. Even if the other realm holds more than one county/holding you can only push a single de jure CB at once. Kingdoms and empires do not provide de jure CBs.
If the claimant wins he or she will gain the contested title, plus some prestige, and the loser will lose prestige.
If white peace occurs the claimant loses prestige and the defender gains prestige.
If the claimant loses he or she loses the claim they pressed, some gold and some prestige, and the successful defender gains some gold and some prestige.
De Jure CB
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.
Change Law CB
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.
Independence CB
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.
Depose Liege CB
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.
Overthrow Ruler CB
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.
Dispute Succession CB
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.
Invasion CB
If you have a claim upon a throne or you're smaller than the realm in question, 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.
Tribal Invasion CB
You'll never get to use this CB unless you mod the game. However, because of the expansion pack of the Sword of Islam, this CB is now available to use for Muslim nations. This CB allows you to invade a nation through might, and if, succeeded, all titles belonging to that realm will be taken in and became yours, Any Muslim ruler can use it. For example: If you are a Sunni ruler who wishes to take control of a Christian or a Shia nation, this CB is almost always available to you. Note that this CB is available only to Muslims and Pagans.
Crusade/Jihad CB
If the Pope has called a crusade, you can declare war on the target, however this system is now changed a bit. When a head of a religion issues a Crusade or Jihad, it is now targeted to a full de jure kingdom, not a small duchy, and the first attacker will always be the head of that particular religion : i.e. if the Pope declares a Crusade for Andalusia, he will be the only attacker at first until other Catholics join him in the war. The same applies with Jihads. The winner of the crusade will always be the ruler who assisted the most during the crusade, effectively giving that ruler a full kingdom.
Conquest CB
This particular CB is available only to Muslim and Pagans only, so if you haven't downloaded the Sword of Islam expansion, you can't use this CB. If 2 Muslim nations are side to side and one decided to expand its border, then a conquest of a county is possible and the winner gets the county. If the attacker loses, he is forced to pay concessions, but if he wins, he will get that county.
Religious War CB
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.
Religious Assistance CB
If another realm in your religion is being targeted by a Religious War CB, you can declare war upon the aggressor. The point of this is merely to prevent your own religion from shrinking. Winning the war will give you 50 piety and prestige, stalemating will lose you 50 piety and 100 prestige, and losing will losing will lose you 150 piety, 200 prestige, and you'll be forced to pay concessions.
Excommunicated Ruler CB
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.
Imperial Reconquest CB
Upon the reformation of Rome in a legacy of Rome DLC game, you will gain a Casus Beli on all territories historically part of the Roman empire by duchy. Winning the war will cause you to vassalize all lords within the duchy, you usurp the capital, and the duchy title itself. Upon reuniting provinces that can be conquered by this you will recieve 100 prestige.