The North Atlantic Treaty Organization (NATO) carried out an aerial bombing campaign against the Federal Republic of Yugoslavia during the Kosovo War. The air strikes lasted from 24 March 1999 to 10 June 1999. The bombings continued until an agreement was reached that led to the withdrawal of the Yugoslav Army from Kosovo, and the establishment of the United Nations Interim Administration Mission in Kosovo, a UN peacekeeping mission in Kosovo. The official NATO operation code name was Operation Allied Force (Serbian: Савезничка сила / Saveznička sila) whereas the United States called it Operation Noble Anvil (Serbian: Племенити наковањ / Plemeniti nakovanj); in Yugoslavia, the operation was incorrectly called Merciful Angel (Serbian: Милосрдни анђео / Milosrdni anđeo), possibly as a result of a misunderstanding or mistranslation.

NATO's intervention was prompted by Yugoslavia's bloodshed and ethnic cleansing of Kosovar Albanians, which drove the Albanians into neighbouring countries and had the potential to destabilize the region. Yugoslavia's actions had already provoked condemnation by international organisations and agencies such as the UN, NATO, and various INGOs. Yugoslavia's refusal to sign the Rambouillet Accords was initially offered as justification for NATO's use of force. Because Russia and China could use their veto within the Security Council to not authorize an external intervention, NATO launched its campaign without the UN's approval, stating that it was inter alia a humanitarian intervention. The UN Charter prohibits the use of force except in the case of a decision by the Security Council under Article 42, under Article 51 or under Article 53. Three days after the commencement of hostilities, on 26 March 1999, the Security Council rejected the demand of Russia, Belarus and India for the cessation of the use of force against Yugoslavia.

By the end of the war, the Yugoslavs had killed 1,500 to 2,131 combatants. 10,317 civilians were killed or missing, with 85% of those being Kosovar Albanian and some 848,000 were expelled from Kosovo. The NATO bombing killed about 1,000 members of the Yugoslav security forces in addition to between 489 and 528 civilians. It destroyed or damaged bridges, industrial plants, hospitals, schools, cultural monuments, and private businesses, as well as barracks and military installations. In total, between 9 and 11 tonnes of depleted uranium was dropped across all of Yugoslavia. In the days after the Yugoslav army withdrew, over 164,000 Serbs and 24,000 Roma left Kosovo. Many of the remaining non-Albanian civilians (as well as Albanians perceived as collaborators) were victims of abuse which included beatings, abductions, and murders. After Kosovo and other Yugoslav Wars, Serbia became home to the highest number of refugees and internally displaced persons (including Kosovo Serbs) in Europe.

The bombing was NATO's second major combat operation, following the 1995 bombing campaign in Bosnia and Herzegovina. It was the first time that NATO had used military force without the expressed endorsement of the UN Security Council and thus, international legal approval, which triggered debates over the legitimacy of the intervention.

A NATO-facilitated ceasefire between the KLA and Yugoslav forces was signed on 15 October 1998, but both sides broke it two months later and fighting resumed. UN Security Council resolution 1160, resolution 1199 and resolution 1203 had been disregarded. When the killing of 45 Kosovar Albanians in the Račak massacre was reported in January 1999, NATO decided that the conflict could only be settled by introducing a military peacekeeping force to forcibly restrain the two sides. Yugoslavia refused to sign the Rambouillet Accords, which among other things called for 30,000 NATO peacekeeping troops in Kosovo; an unhindered right of passage for NATO troops on Yugoslav territory; immunity for NATO and its agents to Yugoslav law; and the right to use local roads, ports, railways, and airports without payment and requisition public facilities for its use free of cost. NATO then prepared to install the peacekeepers by force, using this refusal to justify the bombings.^{[citation needed]}

In its Statement Issued at the Extraordinary Ministerial Meeting of the North Atlantic Council held at NATO Headquarters, Brussels, on 12th April 1999 three weeks after the start of hostilities on 23 March 1999, the collective said that "The Federal Republic of Yugoslavia (FRY) has repeatedly violated United Nations Security Council resolutions. The unrestrained assault by Yugoslav military, police and paramilitary forces, under the direction of President Milosevic, on Kosovar civilians has created a massive humanitarian catastrophe which also threatens to destabilise the surrounding region. Hundreds of thousands of people have been expelled ruthlessly from Kosovo by the FRY authorities. We condemn these appalling violations of human rights and the indiscriminate use of force by the Yugoslav government. These extreme and criminally irresponsible policies, which cannot be defended on any grounds, have made necessary and justify the military action by NATO."

NATO's objectives in the Kosovo conflict were stated at the North Atlantic Council meeting held at NATO headquarters in Brussels on 12 April 1999:

Operation Allied Force predominantly used a large-scale air campaign to destroy Yugoslav military infrastructure from high altitudes. After the third day of aerial bombing, NATO had destroyed almost all of its strategic military targets in Yugoslavia. Despite this, the Yugoslav army continued to function and to attack Kosovo Liberation Army (KLA) insurgents inside Kosovo, mostly in the regions of Northern and Southwest Kosovo. NATO bombed strategic economic and societal targets, such as bridges, military facilities, official government facilities, and factories, using long-range cruise missiles to hit heavily defended targets, such as strategic installations in Belgrade and Pristina. The NATO air forces also targeted infrastructure, such as power plants (using the BLU-114/B "Soft-Bomb"), water-processing plants and the state-owned broadcaster. The Dutch foreign minister Jozias van Aartsen said that the strikes on Yugoslavia should be such as to weaken their military capabilities and prevent further humanitarian atrocities.