The North Atlantic Treaty Organization also called the North Atlantic Alliance, is an intergovernmental military alliance between 30 member states – 27 European, one transcontinental, and two North American. Established in the aftermath of World War II, the organization implements the North Atlantic Treaty, which was signed in Washington, D.C. on 4 April 1949.[3][4] NATO is a system of collective security: its independent member states agree to defend each other against attacks by third parties. During the Cold War, NATO operated as a check on the perceived threat posed by the Soviet Union. The alliance remained in place after the dissolution of the Soviet Union and has been involved in military operations in the Balkans, the Middle East, South Asia, and Africa.