Casus belli is a Latin phrase, meaning "cause of war", "reason for war", or "justification for war". The need for a casus belli before declaring war exists in all versions of Europa Universalis. In all versions of the game, you can start a war if you have a casus belli. In some games it is possible to start a war without one - but you will face negative consequences.

For more detail on the mechanics and tactics, see the following articles:

Europa Universalis: The Musical contained a song called casus belli. This, in turn inspired a mod for EU4 called casus belli

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