The North Atlantic Treaty Organization (NATO) was officially formed on 4 April 1949 with the signing of the Washington Treaty. The purpose of the alliance was to form a collective defense against potential aggressors to Western Europe and the North Atlantic region. Since its inception and as a part of collective defense, the Alliance has maintained a constant state of readiness against an air attack through its air policing operations. During the Cold War fighter-interceptor aircraft stood ready to repel a Soviet attack from the air. These fighter-interceptors were the first line of defense against Russian bombers and attack aircraft staging a conventional or tactical nuclear strike. Ultimately, air policing ensured the sovereignty of NATO members' airspace and was expected to be one of the first counters to the start of World War III.