A term originating from Latin meaning cause of war, a casus belli (commonly abbreviated to CB) is a reason or justification that a country gives when issuing a declaration of war. In the game, a country with a Casus Belli on another country is deemed to have a valid cause for war against another nation, and the casus belli that is chosen when declaring war determines the wargoal.
It is possible to declare war without a casus belli, but doing so will give significant penalties:
In addition, only Superiority wargoals are available for wars declared without a casus belli, which triples the warscore cost of all demands. Therefore it is highly advisable to have a CB when going to war. A CB can be obtained by various diplomatic actions, most costing some political influence.
Types of CB
Casus belli come in a variety of forms, including:
- Claims: The most common casus belli, a country is considered to have a casus belli against another country if it has a claim on a territory that the other country owns.
- Supporting Rebels: A country is always considered to have a casus belli against someone who supports rebels inside it.
- Insults: A country will receive a time limited casus belli against a country that has sent an insult to it.
- Broken Subject Statuses: A country will receive a time limited casus belli against a country that breaks a subject relationship with them.
- Broken Alliances: A country will receive a time limited casus belli against a country that dishonors its alliance towards them.
- Demanding Military Access: A country can declare war on another country if it refuses to grant military access
- Events: Certain special events allow a country to immediately declare war using cbs that they might not otherwise be able to get.
- Inventions: Some casus bellis are unlocked after adopting certain inventions.
Every war has a certain war goal defined by the casus belli used to start the war. Not fulfilling it will give ticking warscore over time to the defending side, while a fulfilled war goal will instead give ticking war score to the attacking side. There are several different war goals in the game:
- Conquest: If a country has a claim on a province held by another country, a conquest war can be declared over that province. The war goal is to completely control the province, and the attacker will only gain ticking warscore if they occupy every territory in the province, while the defender only needs to make sure they control at least one territory to gain ticking warscore for their side. A conquest war will automatically end if the entire war goal is held by either party for 1 year + 180 days per rank - either a white peace if the defender has managed to completely hold it, or ceding the contested province to the attacker if they have managed to occupy it for long enough. Demanding territory with this war goal has a -25% warscore cost reduction for the attacker, even for unclaimed territories.
- Show Superiority: This war goal is always available, even without a casus belli, and the objective is to show superiority in battle. Once over 10 war score from battles is achieved, the extra ticking war score will begin to apply. Demanding territory with this war goal has a +150% (2.5 times) warscore modifier for the attacker, making it suboptimal to use this casus belli for conquest unless it is impossible to obtain a claim.
- Show Naval Superiority: This war goal is available as long as both the potential attacker and defender have a coast, even without a casus belli, and the objective is to show superiority in naval battles. Ticking war score will apply if one side is blockading enemy territory while the other is not. Demanding territory with this war goal has a +1000% (11 times) warscore modifier for the attacker, making it very inefficient to use this casus belli for conquest.
- Imperial Challenge: This war goal is unlocked by the Winning Land by the Spear Oratory invention and can be used on any major power or great power. It is also used by the special event A Chance to Strike for the 20px Parthian invasion of the Seleukid Empire and given as a reward against Carthage for one of the tasks of the Roman mission tree The First Provincia. Unlike in most wars, any territories belonging to either warleader that are occupied in a war using the Imperial Challenge wargoal are immediately annexed (by the other warleader) without any need to demand it in a peace, at the cost of 0.75 aggressive expansion (adjusted by the aggressive expansion impact modifier) and 0.30 war exhaustion per territory. This allows for the conquest of much more territory than would be possible in a normal war, if the country can handle the instability from the resulting high aggressive expansion war exhaustion, though note that the instant annexation does not apply for any secondary participants (including subjects and allies). Warscore is generally less important than the actual military situation and control of territory, particularly as there is limited opportunity to get warscore from occupation, but as with other superiority wargoals, ticking war score will begin to apply once one side as over 10 war score from battles. Making demands with this war goal, while not as relevant, has a +60% warscore modifier for both the attacker and defender.
- Legacy of Alexander: This war goal is identical to the Imperial Challenge wargoal, except that it is available for any of the Diadochi states ( Egypt, Macedon, Thrace, the Seleukid Empire, or the Antigonid Kingdom) to use against each other, as well as against any of the western Macedonian satrapies ( Cappadocia if it has Macedonian primary culture, and Caria, Sardis, and Phrygia (Satrapy) if they were released from the The Antigonid Cause Wavers event chain). The casus belli is unlocked once the The Wars of the Diadochi event has triggered a few months into the game, and expires after receiving the The Epigoni on the death of their first ruler (or the second for Antigonid Kingdom), which allows them to easily expand against each other in the early game before the situation stabilizes. This wargoal is also the wargoal used in various events that start the Fourth War of the Diadochi with the Antigonid Kingdom usually ending up at war with all of the other Diadochi.
- Independence: This war goal can be used by subject states on their overlord, and is automatically used in rebellions from low province loyalty. The objective is to control the capital province of the attacker (always the rebelling country), with ticking warscore accumulating for the attacker if they maintain control over every territory in the province, while the defender will gain warscore as long as they control at least one territory there. Like conquest wars, independence wars will automatically end if the entire war goal is held by either party for 1 year + 180 days per rank. Demanding territory with this war goal has a -25% warscore cost reduction for the attacker.
- Gain Military Access: This war goal is automatically used when declaring war from refusing a military access demand, and the objective is to completely control the capital province. As with all control wargoals, ticking warscore will accumulate for the attacker if they occupy every territory in the province, while the defender gains warscore as long as they control at least one territory there. No demands can be made on either side if this wargoal is active, but the loser will be forced to grant military access for the duration of the truce to the victor.