Rules of War - Lawful And Unlawful Wars

The only type of war recognized by the United Nations as lawful is one fought in self-defense. The rules of warfare are not suspended, however, or otherwise rendered inapplicable merely because the grounds for fighting a particular war are unlawful. In an illegal war both the aggressor and other belligerents must still comport their behavior with the international customs, practices, and conventions of war. At the same time, some authority suggests that one belligerent may disregard certain rules of war in reprisal for its enemy's disregard of the same rules. Such reprisals have a tendency to spiral downward, however, with each act of retaliation straying further from the lawful norms of warfare.