Jus ad bellum and jus in bello

International humanitarian law (IHL), or jus in bello, is the law that governs the way in which warfare is conducted. IHL is purely humanitarian, seeking to limit the suffering caused. It is independent from questions concerning the justification or reasons for war or its prevention, which are covered by jus ad bellum.

The clear distinction between jus in bello and jus ad bellum is comparatively recent. The terms did not become common in debates and writings about the law of war until a decade after World War II. The concepts they cover certainly featured in legal debate before then, but without the clear distinction the adoption of these terms has provided.

The purpose of IHL is to limit the suffering caused by war by protecting and assisting its victims as far as possible. It therefore addresses the reality of a conflict without considering the reasons for or the legality of resorting to force. It regulates only those aspects of the conflict that are of humanitarian concern. This is what is known as jus in bello (law in war). Its provisions apply to the warring parties irrespective of the reasons for the conflict and whether or not the cause upheld by either party is just.

Jus ad bellum (law on the use of force) or jus contra bellum (law on the prevention of war) seeks to limit resort to force between states. Under the UN Charter, states must refrain from the threat or use of force against the territorial integrity or political independence of another state (Art. 2, para. 4). Exceptions to this principle are action taken in self-defence or following a decision adopted by the UN Security Council under Chapter VII of the UN Charter.

In cases of international armed conflict, it is often hard to determine which state is guilty of violating the UN Charter. The application of humanitarian law does not involve denouncing the guilty parties as that would be bound to arouse controversy and paralyse implementation of its provisions since each adversary would claim to be a victim of aggression. Moreover, IHL is intended to protect war victims and their fundamental rights, no matter which party they belong to. That is why jus in bello must remain independent of jus ad bellum or jus contra bellum.