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A brief war fought in 1999 between NATO and Serbia, the main remnant of the former Yugoslavia, over the status of the Yugoslavian province of Kosovo. In 1998, Serbian president Slobodan Milosevic sent Serbian troops to take back areas of Kosovo controlled by ethnic Albanian guerrillas, triggering a NATO air campaign in 1999. The war ended when Serbia agreed to a peace agreement sponsored by the United Nations.
Note: The Kosovo War was NATO's first military engagement. It was controversial in part because NATO was not repelling an attack on member states but intervening in what was arguably an internal affair.