What are jus ad bellum and jus in bello?
Extract from ICRC publication "International humanitarian law: answers to your questions"
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.
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 (law on the use of force or law on the prevention of war).
On the prohibition of war
Until the end of the First World War, resorting to armed force was regarded not as an illegal act but as an acceptable way of settling differences. In 1919, the Covenant of the League of Nations and, in 1928, the Treaty of Paris (Briand-Kellogg Pact) sought to outlaw war. The adoption of the United Nations Charter in 1945 confirmed the trend: The members of the Organization shall abstain, in their international relations, from resorting to the threat or use of force (...). When a State or group of States is attacked by another State or group of States, however, the UN Charter upholds the right to individual or collective self-defence. The UN Security Council, acting on the basis of Chapter VII of the Charter , may also decide on the collective use of force. This may involve: -coercive measures aimed at restoring peace against a State threatening international security; -peace-keeping measures in the form of observer or peacekeeping missions. A further instance arises within the framework of the right of peoples to self-determination: in resolution 2105 (XX) adopted in 1965, the UN General Assembly recognizes the legitimacy of the struggle waged by peoples under colonial domination to exercise their right to self-determination and independence (...).