The Kosovo Liberation Army (KLA; Albanian: Ushtria lirimtare e Kosovs [utija tliimta ksvs], UK) was an ethnic Albanian separatist militia that sought the separation of Kosovo, the vast majority of which is inhabited by Albanians, from the Federal Republic of Yugoslavia (FRY) and Serbia during the 1990s. Albanian nationalism was a central tenet of the KLA and many in its ranks supported the creation of a Greater Albania, which would encompass all Albanians in the Balkans, stressing Albanian culture, ethnicity and nation. Throughout its existence the KLA was designated as a terrorist group by FRY.Military precursors to the KLA began in the late 1980s with armed resistance to Yugoslav police trying to take Albanian activists in custody. By the early 1990s there were attacks on police forces and secret-service officials who abused Albanian civilians. By mid-1998 the KLA was involved in frontal battle though it was outnumbered and outgunned. Conflict escalated from 1997 onward due to the Yugoslavian army retaliating with a crackdown in the region which resulted in violence and population displacements. The bloodshed, ethnic cleansing of thousands of Albanians driving them into neighbouring countries and the potential of it to destabilize the region provoked intervention by international organizations, such as the United Nations, NATO and INGOs. NATO supported the KLA and intervened on its behalf in March 1999.In September 1999, with the fighting over and an international force in place within Kosovo, the KLA was officially disbanded and thousands of its members entered the Kosovo Protection Corps, a civilian emergency protection body that replaced the KLA and Kosovo Police Force, as foreseen in United Nations Security Council Resolution 1244. The ending of the Kosovo war resulted in the emergence of offshoot guerilla groups and political organisations from the KLA continuing violent struggles in southern Serbia (19992001) and northwestern Macedonia (2001), which resulted in peace talks and greater Albanian rights. Former KLA leaders also entered politics, some of them reaching high-ranking offices.

The KLA received large funds from Albanian diaspora organizations. There have been allegations that it used narcoterrorism to finance its operations. Abuses and war crimes were committed by the KLA during and after the conflict, such as massacres of civilians, prison camps and destruction of cultural heritage sites. In April 2014, the Assembly of Kosovo considered and approved the establishment of a special court to try cases involving crimes and other serious abuses allegedly committed in 19992000 by members of the KLA. In June 2020 the Kosovo Specialist Chambers and Specialist Prosecutor's Office filed indictments for crimes against humanity and war crimes against a number of former KLA members, including the former president of Kosovo Hashim Thai.

The Kosovo War was an armed conflict in Kosovo that started 28 February 1998 and lasted until 11 June 1999. It was fought by 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 formed in the early 1990s to fight against Serbian persecution of Kosovo Albanians. The KLA 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.After attempts at a diplomatic solution failed, NATO intervened, justifying the campaign as a "humanitarian war". This precipitated a mass expulsion of Kosovar Albanians as the Yugoslav forces continued to fight during the aerial bombing of Yugoslavia (March–June 1999). By 2000, investigations had recovered the remains of almost three thousand victims of all ethnicities, and in 2001 a United Nations administered Supreme Court, based in Kosovo, found that there had been "a systematic campaign of terror, including murders, rapes, arsons and severe maltreatments", but that Yugoslav troops had tried to remove rather than eradicate the Albanian population.The war ended with the Kumanovo Treaty, with Yugoslav and Serb forces agreeing to withdraw from Kosovo to make way for an international presence. The Kosovo Liberation Army disbanded soon after this, with some of its members going on to fight for the UÇPMB in the Preševo Valley and others joining the National Liberation Army (NLA) and Albanian National Army (ANA) during the armed ethnic conflict in Macedonia, while others went on to form the Kosovo Police. After the war, a list was compiled which documented that over 13,500 people were killed or went missing during the two year conflict. The Yugoslav and Serb forces caused the displacement of between 1.2 million to 1.45 million Kosovo Albanians. After the war, around 200,000 Serbs, Romani, and other non-Albanians fled Kosovo and many of the remaining civilians were victims of abuse.The NATO bombing campaign has remained controversial. It did not gain the approval of the UN Security Council and it caused at least 488 Yugoslav civilian deaths, including substantial numbers of Kosovar refugees.