A more thorough explanation:

Definition: A just war; one that is considered morally and legally justifiable, such as a war against an aggressive, totalitarian regime.

Under Roman law, before war could be declared, a group of priests called the fetiales had to certify to the Senate that just cause for war existed. Medieval theologian-jurists like Thomas Aquinas debated the circumstances that justified war, with some supporting the idea of a just war against non-Catholics. However, with the adoption of the U.N. Charter, the concept of bellum justum has lost its legal significance. The Charter outlaws the use of force except in self-defense.

Examples: A war against a country that is committing genocide or invading another country without provocation could be considered a bellum justum. However, a war for economic gain or to expand territory would not be considered justifiable.

This term is important in international law because it helps to establish guidelines for when it is acceptable to go to war. It emphasizes the importance of considering the moral and legal implications of using force and encourages countries to seek peaceful solutions whenever possible.