In May 2000 Dragia Krsmanovich, the Serbian Public Prosecutor, began the indictment for war crimes of the leaders of the United States of America (USA), the United Kingdom (UK), France, and Germany, and the former North Atlantic Treaty Organisation (NATO) Secretary-General Javier Solana. These charges, laid in the Serbian Supreme Court, related to alleged violations by NATO forces of the Geneva Convention on the conduct of war. These violations were alleged to have occurred in the course of NATO attacks upon Serbia during 1999, operations which were undertaken to persuade the Yugoslav Government to comply with United Nations Security Council Resolutions 1160, 1199 and 1203. These resolutions had required Yugoslavia's withdrawal from the province of Kosovo. During enforcement operations conducted from 24 March to 8 June 1999 NATO was alleged to have breached the Convention by using cluster bombs, and by attacking civilians, residential areas, and non-military targets. Ironically, Yugoslav President Slobodan Milosevic and four other Yugoslav Serbs have themselves since been indicted by the Chief Prosecutor of the International Criminal Tribunal for the former Yugoslavia, also for war crimes and crimes against humanity. In this case these crimes were allegedly committed against the Kosovo Albanians. A situation such as the collapse of the former Federal Republic of Yugoslavia is one fraught with dangers and difficulties for the international community. Not least of the difficulties have been legal, including the prosecution of political and military leaders by international courts. As with the trial of Libyan suspects in the Lockerbie case, the development of rules and procedures of international criminal trials have been influenced more by political considerations than by legal. But we can say with some confidence that an international criminal legal system is slowly developing. This is consistent with a growing sophistication in public international law, originally confined to state-to-state relations. It was unusual to prosecute war leaders in international courts, but they were always liable to sanctions of some kind, however ineffective. Perhaps more problematic however is the question whether the Serbian public prosecutor had a good case. Was NATO guilty of war crimes? The most serious war crime was traditionally levying war without just cause. Although the basis of Serbia's action against NATO and the allies was more narrowly focused, alleging the illegal use of cluster bombs, and attacking civilians, residential areas, and non-military targets, the whole basis of NATO's actions may possibly be challenged. For most purposes, the law of war may be divided into two parts: the legitimacy of the resort to force, and the rules governing the conduct of hostilities, often called jus ad bellum and jus in bello respectively. Both changed markedly in the course of the twentieth century. This paper will consider the broader question of whether NATO's actions in attacking Yugoslavia were lawful (jus ad bellum). It will also look briefly at some of the legal ramifications of the more recent USA-led global war on terrorism, which also raises questions of the legality of the use of force in non-traditional military operations. The narrower question of a breach of the laws of war, by attacking civilian targets (jus in bello), will be left to other writers.