International humanitarian law, 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 about the justification or reasons for war, or its prevention, 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 did feature in legal debate before then, but without the clear distinction the adoption of the terms has brought about.
The purpose of international humanitarian law is to limit the suffering caused by war by protecting and assisting its victims as far as possible. The law therefore addresses the reality of a conflict without considering the reasons for or legality of resorting to force. It regulates only those aspects of the conflict which are of humanitarian concern. It 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.
The ius ad bellum (law on the use of force) or ius 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 provided in case of self-defence or following a decision adopted by the UN Security Council under chapter VII of the UN Charter.
In the case of international armed conflict, it is often hard to determine which State is guilty of violating the United Nations Charter. The application of humanitarian law does not involve the denunciation of guilty parties as that would be bound to arouse controversy and paralyse implementation of the law, 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 to which party they belong. That is why jus in bello must remain independent of jus ad bellum or jus contra bellum.