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As of the Spring 2018 Update, Casus Belli can be used when declaring a Joint War with another civilization. Most Casus Belli are unlocked by researching specific civics, as detailed below.
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As the game goes on and you enter new eras, the penalties increase on both Surprise War and the various Casus Belli. The alternative to using a Casus Belli is the Surprise War, which will give you a much larger warmonger/grievances penalty than the Casus Belli, except in the Ancient Era in which there is no warmonger penalty. 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 ( Grievances in Gathering Storm).
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