The North Atlantic Treaty Organization (NATO), also known as the North Atlantic Alliance, is a military alliance established by the signing of the North Atlantic Treaty on 4 April 1949. NATO, which is headquartered in Brussels, Belgium, constitutes a system of collective defense whereby its member states agree to mutual defense in response to an attack by any external party. Since the fall of the Soviet Union, NATO has been quickly expanding beyond its traditional membership base in North America and Western Europe as it struggles to find relevance in the post-Cold War world.