A more thorough explanation:

Justifiable war, also known as bellum justum, refers to a war that is considered morally and legally justifiable. This type of war is often waged against an aggressive, totalitarian regime.

For example, the United States' involvement in World War II is considered a justifiable war because it was fought against the aggressive actions of Nazi Germany and Imperial Japan.

Historically, the concept of a just war has been debated by theologians and jurists. In Roman law, the fetiales, a group of priests, had to certify to the Senate that just cause for war existed before it could be declared. Some medieval theologian-jurists supported the idea of a just war against non-Catholics.

However, with the adoption of the United Nations Charter, the concept of a justifiable war has lost its legal significance. The Charter prohibits the use of force except in self-defense.