Casus belli is a Latin term meaning case of war. In the game, a country with a Casus Belli (commonly abbreviated to "CB") on another country is deemed to have a valid cause for war. Declaring war without a Casus Belli will cost the attacker 2 Stability and give additional Aggressive Expansion. Casus belli can be produced for 200 (180 with level 0 invention) oratory power in the diplomacy window.
When going to war with another country it is highly advisable to have a casus belli.
Every war has a certain war goal defined by the casus belli used to start the war. Not fulfilling it will give warscore over time to the defending side, while a fulfilled war goal gives ticking war score to the attacking side. There are three different War Goals in the game:
- Conquest: If you have a Claim on a Province held by another country you can declare conquest war over that Province. In that case holding the entire enemy province selected will be the war goal. A conquest war will automatically end if the entire war goal is held by either party for 1 year + 180 days per Rank. Conquering provinces costs 25% less warscore.
- Show Superiority: The War goal is to show superiority in battle. Once over 10 war score from battles is achieved the extra ticking war score starts applying. Conquering provinces costs triple warscore.
- Show Naval Superiority: The War goal is to show superiority in naval battles. Once over 10 war score from battles is achieved the extra ticking war score starts applying. Having more blockades than your enemy will also contribute. Conquering provinces costs 11 times as much warscore.
Casus belli come in a variety of forms including:
- Claim: A country is considered to always have a casus belli against a country if it has a claim on a city that the other country owns.
- Supporting Rebels: A country is always considered to have a casus belli against someone who supports rebels inside it.
- Insult: A country will receive a time limited casus belli against a country that has sent an insult to it.
- Guarantee: A country will receive a time-limited casus belli against a country that attacks someone guaranteed by them.
- Broken Subject Status: A country will receive a time limited casus belli against a country that breaks a subject relationship with them.
- Broken Alliance: A country will receive a time limited casus belli against a country that dishonors its alliance towards them.