As the Allied countries and the Soviet Union fell into the Cold War, the Domino Theory became a popular political mindset. The Domino Theory stated that if the Soviets were allowed to take over one country and institute communism, then nearby countries would fall like dominoes to the evils of communism. The Domino Theory pushed the U.S. to strengthen its alliances with existing partners in Western Europe. The United States focused on a policy of containment, believing containing communism to areas where it already existed would be the best approach to deal with Soviet threats of communist expansion. The United States and other Allied countries formed NATO to deal with Soviet threat of force and communism. NATO was built upon The Western Union Defense Organization. In 1999, NATO launched airstrikes against Yugoslavia to put an end to the ethnic cleansing that was occurring and help instill peace. The Soviet response to NATO was the Warsaw Pact. NATO was different from the Warsaw Pact because NATO was a peaceful alliance of countries.