Definition - What does Casus Belli mean?
Casus belli is a Latin term that means "a case for war." In the context of the law, casus belli is used in international law to describe events that justify a war. Wars often occur, but it is the cause of a war that can determine whether or not it is justified. For example, if the war is caused by self-defense, it may be justified according to international law.
Justipedia explains Casus Belli
Causes of war that are not considered justified and sanctioned by international law can result in prosecution for war crimes. A country that is found guilty of war crimes can face a number of punishments.
Defense of an ally that is under an attack is another casus belli that is considered justified. Another situation that can be qualified as a legally acceptable justification for war is if the United Nations sanctions the war.