Which principle is related to the necessity of self-defense in the Law of War?

The principle related to the necessity of self-defense in the Law of War is necessity. This principle asserts that the use of force must be required to achieve a legitimate military objective. In the context of self-defense, it underscores that a party may resort to force only when it is essential to address an imminent threat and that the response is appropriate to counter that specific threat.

Necessity ensures that military actions are not taken lightly and are grounded in the need to protect oneself or one's troops from any aggression. This principle helps to maintain a balance within the framework of the Law of War, ensuring that force is used judiciously and only when absolutely required for survival or protection.

The other options, while relevant to the overall discussions about war and military engagement, focus on different aspects. For example, proportionality relates to ensuring that the level of force used is appropriate to the military advantage gained, and legitimacy pertains to the justification and legal grounding for actions taken in war. However, necessity directly addresses the fundamental question of whether the use of force is warranted in self-defense situations.