There was a time during the Cold War where the Soviet Union attempted to join NATO. NATO or the North Atlantic Treaty Organization, is an intergovernmental military alliance that formed in April 4th of 1949. The alliance originally only had 12 countries. However, some consider NATO to have started in 1948 when the Treaty of Brussels was signed as a mutual defense treaty against the Soviet Union. This treaty was signed by Belgium, France, Luxembourg, the Netherlands, and the United Kingdom. However, this defense pact did not include the United States and future members of NATO. The 1948 Treaty was signed by:
Prince Charles of Belgium, as the reigning Prince Regent of Belgium
Joseph Bech, Luxembourgish Minister of Foreign Affairs
Gaston Eyskens, Belgian Minister of Finance
Carel Godfried Willem Hendrik baron van Boetzelaer van Oosterhout, Dutch Minister of Foreign Affairs
Ernest Bevin, Secretary of State for Foreign Affairs of the United Kingdom
Jean de Hautecloque, Ambassador Extraordinary and Plenipotentiary of the French Republic in Brussels
Robert Als, Envoy Extraordinary and Minister Plenipotentiary of Luxembourg in Brussels
Baron Binnert Philip van Harinxma thoe Slooten, Ambassador Extraordinary and Plenipotentiary of the Netherlands in Brussels
George William Rendel, Ambassador Extraordinary and Plenipotentiary of His Britannic Majesty in Brussels
It was ratified in Belgium April 3rd, 1948.
NATO predated the Warsaw Pact (which will be discussed in a future article) by about 6 years. The USSR offered to join NATO even though it was pretty much created as an alliance against the USSR & the Red Army. Officially, this was a defensive alliance, however, it was supposed to defend against any Marxist-Leninist socialist movements. Two important Soviet politicians, Lavrentiy Beria and Vyacheslav Molotov, had an interesting idea of becoming part of NATO. The plan would involve the USSR giving up East Germany and reunifying Germany as a neutral state when the Soviet Union becomes a member of NATO. The likelihood of the USSR being accepted into NATO was slim and the politicians knew this. However, the Politburo (Highest worker's committee) did prepare in case NATO said yes. The idea was also based around the Stalin Note, a note from Premier of the Soviet Union Joseph Stalin delivered to the Western Allies on March 1952 that proposed German reunification if German remained neutral.
The obvious answer was 'no' and the Soviet Union responded by creating their own defensive alliance in the form of the Warsaw Pact. The request was specifically rejected by the United States and the United Kingdom.