Casus belli

From EU3 Wiki

Casus belli comes from latin, meaning justification for war. In the game a country with a casus belli (commonly known as just "CB") on another country is deemed to have a valid cause of war. A country with a CB will lose less stability and reputation by declaring war compared to the same situation without the CB.

You can get casi belli (plural) in several ways:

• If another country owns a province your nation considers one of its cores, you have a CB against that nation. Be aware that core claims go away after a good time. It happens when you lose a core on that province, usually after 50 years or as a consequence of a certain peace resolution. A flag shows up when the CB is about to end.
• Sometimes an event might pop up giving a free CB (a diplomatic incident or a boundary dispute).
• Should you warn a country and it still goes to war with one of its neighbours you will also gain a CB.
• If you have Deus Vult national idea you have a free casus belli on anybody who doesn't share your religion.
• If you insult a country, it gets a CB against you.
• When your ally is attacked by somebody or attacks somebody you have a casus belli on that country.

There are other ways too.