The law of armed conflict (jus in bello) applies equally to all parties to an armed conflict, irrespective of the law governing the resort to force (jus ad bellum or jus contra bellum). In other words, an aggressor State must comply with the law of armed conflict in the same way as the State defending itself. What are the reasons for this strict separation between jus in bello and jus ad bellum, and does the separation make sense in contemporary conflicts?