The Kosovo War was an armed conflict in Kosovo that lasted from 28 February 1998 until 11 June 1999. It was fought between the forces of the Federal Republic of Yugoslavia (i.e. Serbia and Montenegro), which controlled Kosovo before the war, and the Kosovo Albanian rebel group known as the Kosovo Liberation Army (KLA). The conflict ended when the North Atlantic Treaty Organization (NATO) intervened by beginning air strikes in March 1999 which resulted in Yugoslav forces withdrawing from Kosovo.
The KLA was formed in the early 1990s to fight against Serbian persecution of Kosovo Albanians, with the goal of uniting Kosovo into a Greater Albania. It initiated its first campaign in 1995 when it launched attacks against Serbian law enforcement in Kosovo. In June 1996, the group claimed responsibility for acts of sabotage targeting Kosovo police stations, during the Kosovo Insurgency. In 1997, the organisation acquired a large amount of arms through weapons smuggling from Albania, following a rebellion in which weapons were looted from the country's police and army posts. In early 1998, KLA attacks targeting Yugoslav authorities in Kosovo resulted in an increased presence of Serb paramilitaries and regular forces who subsequently began pursuing a campaign of retribution targeting KLA sympathisers and political opponents; this campaign killed 1,500 to 2,000 civilians and KLA combatants, and had displaced 370,000 Kosovar Albanians by March 1999.
On 20 March 1999, Yugoslav forces began a massive campaign of repression and expulsions of Kosovar Albanians following the withdrawal of the OSCE Kosovo Verification Mission (KVM) and the failure of the proposed Rambouillet Agreement. In response to this, NATO intervened with an aerial bombing campaign that began on March 24, justifying it as a "humanitarian war". The war ended with the Kumanovo Agreement, signed on 9 June, with Yugoslav and Serb forces agreeing to withdraw from Kosovo to make way for an international presence. NATO forces entered Kosovo on June 12.