Casus belli - CK3 Wiki A ruler must have a valid casus belli in order to be able to declare war upon another ruler, can only gain from a war what the casus belli specifies, and the war is ended if the requirements for its casus belli are no longer met Each casus belli will cost either Prestige or Piety
What is the easiest way to obtain casus belli? : r ck3 - Reddit Near the bottom of the Scholar Perk tree is the "Sanctioned Loopholes" perk This allows you to cash in Piety for claims on your fellow Ecumenical Christians (typically, only Counties and Duchies)
Casus Belli :: Crusader Kings III General Discussions You need a Claim on territory, or if you're near a domain of a different religion then you can claim Holy War if you have the large amount of Piety to spend on it Plus there are limits regarding how much you can do at once depending on your level of Religion displayed in the Piety tooltip