The attacks on New York and Washington on 11 September 2001 and the subsequent “war on terrorism” have suddenly brought international humanitarian law into the limelight and again highlighted the relationship between the causes of a conflict on one hand and the respect for rules on the conduct of hostilities and protection of victims of war on the other. The article traces the history of rules limiting violence and the interdiction of recourse to war. Despite the general banning of war in the Charter of the United Nations, the application of the jus in bello remains independent of the causes of war, even in a war against aggression, and every discriminatory application of international humanitarian law has to be rejected. There are imperious reasons for maintaining the principle of egality of the belligerants with repect to the law of war. Whatever its moral and legal intentions, the theory of discriminatory application of the laws and customs of the war leads to the same unacceptable result, namely unrestricted warfare, as the conception that wars of agression are not covered by international humanitarian law. State practice and the Rome Statute of the International Criminal Court, which entered into force on 1st July 2002, confirm the strict separation between jus in bello and jus ad bellum.