A military organization, North Atlantic Treaty Organization, was formed shortly after WWII. It's members at the time included most Western European nations as well as Canada and the US. The purpose of NATO was that in case one of it's members was attacked by the Soviet Union or one of it's allies, the rest of NATO would go to war with that nation who attacked the nation that was part of NATO. Since the end of communist presence, NATO has turned to other problems. NATO has taken part in military actions in the war in Yugoslavia. NATO's actions in that war, including to NATO attacks on Bosnian Serb military targets, helped end the war in Yugoslavia. Even nations that were once part of the Warsaw Pact, such as Poland and Hungary, have joined NATO over the past decade.
NATO had taken part in military operations against Yugoslavia during the Kosovo conflict, mainly by launching air raids over Yugoslavia.