Casus Belli is a new gameplay mechanic introduced in Civilization VI that allows you, after you meet certain prerequisites, to justify the wars you declare and get fewer warmonger penalties. The alternative to using a Casus Belli is the Surprise War, which will give you a much larger warmonger penalty than the Casus Belli, except in the Ancient Era in which there is no penalty. As the game goes on and you enter new eras, the warmonger penalties increase on both Surprise War and Casus Belli.