Abstract: War is at the core of the efforts to submit the use of military force in international relations to legal rules. For millennia the decision to wage war was not subject to any legal restrictions. Furthermore, war was regarded as a legitimate means of policy, its foremost aim changing territorial boundaries. In the early years of the twentieth century, nearly all states agreed on a ban on the use of force with the explicit exception of self- defense, thus legally accepting only peaceful changes to the status quo. Situations may occur in which the use of force might be believed to be legitimate by moral standards although illegal. Thus, bound between the dichotomy of security and justice, the requirements of legality and legitimacy might not always coincide in international law.