A casus belli (CB) is an internationally acceptable justification for declaring war on another country. When a country declares war without having a CB, it takes more severe hits to badboy and stability.
Seeing Casus Belli
There are several ways to determine the casus belli a country has in the game. On the diplomacy screen is a listing of all diplomatic relationships a country has, including casus belli. Permanent CBs are listed together. Temporary CBs are listed after the permanent ones.
You can see CBs graphically in diplomacy map mode by selecting a country. Other countries it has a CB on will be shown in red, with "lightning" over their crown.
Effects of Casus Belli
The primary effect of a casus belli is seen only when a country declares war on another. If it has no casus belli, it will take a larger hits to stability and badboy.
In addition to the effects on DoWs, whenever a country has a CB on another, and it does not also have a royal marriage, relations between the two countries will decline slightly each month.
Duration of Casus Belli
There are two kinds of casus belli: permanent and temporary.
Temporary CBs all have an expiration date. All temporary CBs have a specific event in the game's history which caused them, and which their expiry date is related to. You can see the date when they expire by mousing over them in the diplomacy screen, and getting a tooltip. (The exact day of expiration is not visible in-game; you can see it in a save file.)
Permanent CBs do not have an expiration date. They are based on ongoing conditions in the game; so long as those conditions are true, the casus belli will exist. It ceases immediately if the condition causing it ceases. Note that "permament" is a bit of a misnomer, since some permanent CBs don't usually last very long.
Causes of Casus Belli
There are many ways in which a country can get a casus belli on a second country.
Causes of Permanent CBs
The following conditions create permanent CBs:
- Core province ownership: a country has a CB on any country which owns one or more of its core provinces.
- War with the Papal States: all Catholic and Counterreform Catholic countries have a CB on all countries which are currently at war with the Papal States.
- Ownership of Rome: if a country other than the Papal States owns Rome, all Catholic and Counterreform Catholic countries have a CB on it.
- Embargo: a country has a CB on any country which has imposed a trade embargo on it.
- Islamic heresy: all Shi'ite countries have a CB on all Sunni countries, and vice versa.
- Christian heresy: all Counterreform Catholic countries have a CB on all Protestant and Reformed countries, and vice versa, until the Edict of Tolerance.
- Defense of the Faith: any country which has obtained the mantle of Defender of the Faith gets a CB on any other country which is at war with any country of its faith. Note that EU2 is buggy and this effect does not happen if the religion involved is Counterreform Catholic or Reformed.
- Badboy: when a country is over its badboy limit, it gives a CB to all other countries.
Causes of Temporary CBs
The following occurances give temporary CBs:
- Force annexation: a country that annexes another one by force gives a two year CB to each other country in the world whose relations with it are < -150.
- Scenario-defined CBs: some scenarios define some countries as having CBs on others. For example, England in the 1419 scenario has "temporary" CBs on Eire and Scotland, lasting until 1819. Spain is given CBs on all pagan and muslim countries.
- Events: many scripted events give CBs of various lengths. Random events also have some CBs as results, for times ranging from 6 months to 10 years.
- Province defection: when a province defects from one country to a second country, the first country gets a 2 year CB on the second.
- Voluntary religion change: when a country changes its state religion voluntarily, all other countries of its former religion get a 1 year CB.
- Dishonoring an alliance: when a country dishonors an alliance call to war, all of its former allies receive a 1 year CB on it.
- Declaring war on a vassal: when a country DoWs a country which is a vassal, its suzerain gets a 1 year CB on the country which declared war.
- Making a separate peace: when a country makes peace separately, all members of its war coalition receive a 1 year CB on it. Note that force-annexation is considered a separate peace.
- Ignoring a warning: if a country has warned a second country and it declares a war within 20 years after the warning, the first country gets a 1 year CB on the second.
- Threatening a guarantee: if a country has guaranteed the independence of a second country, and a third country DoWs the second, the first country gets a 1 year CB on the third.
- Claiming a throne: when a country claims the throne of another, it receieves a 2 year CB.
- Refusal to pay back a loan: you will get a 1 year CB on a country that refuses to pay back a loan given to them.