The Union of Soviet Socialist Republics (U.S.S.R.), commonly known as the Soviet Union, was a pre-War communist state in Eastern Europe and Asia, consisting of 15 Soviet republics. In the Fallout universe, the USSR did not dissolve in 1991, and continued to exist well into the 21st century.
The Soviet Union had a consular service and a foreign service, and had strong consular relations with the United States. There was a Soviet consulate in Los Angeles (the one Natalia's grandfather worked at), suggesting that Soviet-American relations were good enough in 2077 that American and Soviet citizens were able to visit each other's countries. However, in propaganda, the Soviet Union was still being portrayed as a 'communist enemy'.
During the Resource Wars, the Soviet Union was in no direct conflict with the United States, although it is unknown what role they played in the Great War.
↑In line with Marxism, communism has never been implemented because for communism to exist the state must be dissolved and be proceeded by a worldwide socialist phase, the U.S.S.R. was in fact a state socialist system, a so called "dictatorship of proletariat".