When is war justifiable?

The justifiable war tradition centers on a questions that seek to determine whether a particular war might be justifiable.

Conditions that must be met to justify undertaking a war:

1. Last resort. Since the presumption is that war is a great evil, it must be the last resort. All other possible solutions must be exhausted before war can be considered justifiable.

2. Just cause. The cause of war must be so serious and so wrong as to justify war as a response. Claiming to have a just cause and actually having one aren’t the same. Every group that has ever gone to war has claimed that its cause was just.

3. Legitimate authority. Only the proper authority can legitimately undertake war. Of course, the question of who is a proper authority is subject to debate. Are national governments the primary authorities? Is the United Nations the primary authority? Do they share authority?

4. Right purpose. War must have an acceptable purpose. For example, the great medieval theologian Thomas Aquinas said war was justified only if it contributed to the common good. The common good isn’t confined to any one people or nation. War merely for the sake of national interest is unacceptable. For a second example, Martin Luther—and Augustine before him—said violence to defend one’s life wasn’t permissible, but violence to defend the life of one’s neighbor could be justified. Strictly speaking, Luther and Augustine didn’t permit war for “self-defense,” but did allow the possibility of war for “neighbor-defense.”