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:
- Casus belli (Europa Universalis)
- Casus belli (Europa Universalis II)
- Casus belli (Europa Universalis III)
- Casus belli (Europa Universalis IV)