The legitimacy under international law of the 1999 NATO bombing of the Federal Republic of Yugoslavia has been questioned by various parties. The UN Charter is the foundational legal document of the United Nations (UN) and is a primary cornerstone of public international law because UN member states have legally bound themselves to uphold it. At the same time, all member states of the North Atlantic Treaty Organization (NATO) are also member states of the UN, and thus they must also comply with their obligations under the North Atlantic Treaty, the foundational document of NATO.
Supporters of the NATO bombing of Yugoslavia argued that the bombing brought to an end the ethnic cleansing of Kosovo's Albanian population, and that it hastened (or caused) the downfall of Slobodan Milošević's government, which they saw as having been responsible for the international isolation of Yugoslavia, war crimes, and human rights violations.
NATO described the conditions in Kosovo as posing a risk to regional stability. As such, NATO and certain governments asserted they had a legitimate interest in developments in Kosovo, due to their impact on the stability of the whole region which, they claimed, is a legitimate concern of the Organisation.
The UN Charter is legally binding on all United Nations member states, including all members of NATO, because they have each signed it. Article 2(4) of the UN Charter prohibits the use of force by UN member states to resolve disputes, but with two specific exceptions to this general prohibition:
NATO did not have the backing of the United Nations Security Council to use force in Yugoslavia. Further, NATO did not claim that an armed attack occurred against another state. However, its advocates contend that NATO actions were consistent with the United Nations Charter because the UN Charter prohibits unprovoked attacks only by individual states. The principal legal issue remains, however, since NATO as such is not a member state of the UN, whether the member states of NATO, the United States and the European powers that sent armed forces to attack as part of the NATO bombing campaign, violated the UN Charter by attacking a fellow UN member state: (1) in the absence of UN Security Council authorization, and (2) in the absence of an attack or a threat of imminent attack on them.
The United Nations considers NATO to be a "regional arrangement" under UN Article 52, which allows it to deal with matters relating to the maintenance of international peace and security as are appropriate for regional action provided that such arrangements or agencies and their activities are consistent with the Purposes and Principles of the United Nations. However, the UN policy on military intervention by regional arrangements in UN Article 53 states the Security Council can, where appropriate, "utilize such regional arrangements or agencies for enforcement action under its authority. However, no enforcement action shall be taken under regional arrangements or by regional agencies without the authorization of the Security Council."
NATO had justified the actions in Kosovo under Article 4 of its charter, the North Atlantic Treaty, which allows involved parties to consult together whenever political independence or security of any of the parties is threatened. Because the NATO actions in Kosovo were taken after consultation with all members, were approved by a NATO vote, and were undertaken by several NATO members, NATO contends that its actions were in accordance with its charter. Article 4, however, is silent as to the use of force and does not discuss under what circumstances force may be authorized.
Article 5 of NATO's charter calls on NATO members to respond in mutual defense when any NATO member is attacked. It is unclear whether under the NATO charter force may be used in the absence of such an attack. Article 5 has been interpreted as restricting NATO's use of force to situations where a NATO member has been attacked. It has been argued, therefore, that NATO's actions were in violation of the charter of NATO. Critics of this theory argue, however, that the purpose of Article 5 is to require all NATO members to respond when any NATO member is attacked, not to restrict the circumstances under which NATO may choose to use force.
Serbia was threatened by NATO with armed attack if Serbia refused to sign the Rambouillet Agreement, an agreement that Serbia in the end refused to sign. It has been argued that pursuant to Article 52 of the 1980 Vienna Convention on the Law of Treaties, had Serbia signed the Rambouillet Agreement, the agreement would have been void due to the threat or use of force to compel Serbian acceptance.
UN Secretary-General Kofi Annan supported intervention in principle, saying "there are times when the use of force may be legitimate in the pursuit of peace", but was critical of unilateral action by NATO. He argued "under the [UN] Charter the Security Council has primary responsibility for maintaining international peace and security - and this is explicitly acknowledged in the North Atlantic Treaty. Therefore, the Council should be involved in any decision to resort to the use of force."
On the day the bombing started, Russia called for the UN Security Council to meet to consider "an extremely dangerous situation caused by the unilateral military action of the North Atlantic Treaty Organization (NATO) against the Federal Republic of Yugoslavia". However, a draft resolution, tabled jointly by Russia, Belarus and India, to demand "an immediate cessation of the use of force against the Federal Republic of Yugoslavia" was defeated. Among the 15 UN Security Council nations, there were three votes in favour (Russia, China and Namibia) and twelve against, with no abstentions. Argentina, Bahrain, Brazil, Gabon, Gambia, Malaysia, and Slovenia, along with the US, Britain, France, Canada, and Netherlands voted against it.
Rejection of Russia's condemnation amounted to political, but not legal, support of NATO's intervention. After the war ended with the Kumanovo Treaty and the bombing stopped, some argued that the creation on 10 June 1999 of the UN Interim Administration Mission in Kosovo (UNMIK), by Security Council Resolution 1244 (1999), constituted a legal ratification post festum (after the event).
Aside from the above-discussed issue of the legal justifiability of launching the war, jus ad bellum, against Serbia, the NATO bombing campaign has been criticized for exceeding the limits of lawful wartime conduct, jus in bello, under international humanitarian law, such as the Geneva Conventions.
James Byron Bissett, former Canadian ambassador to Yugoslavia, Bulgaria, and Albania, stated that "Canada participated in a series of NATO-sanctioned war crimes against Yugoslavia". He stated that "NATO and the United States claimed that more than 100,000 ethnic Albanians had been killed as the result of Serb genocide". "The forensic experts found fewer than 2,000 graves and many of the people in those graves were Serbs," Bissett said. "There were more civilians killed in Serbia by the NATO bombing campaign."
Noam Chomsky was also highly critical of the NATO campaign and its aerial bombing in particular, where public utilities were bombed in addition to military targets. He viewed the bombing of the Radio Television of Serbia as an act of terrorism.