The Soviet Union or U.S.S.R. (Union of Soviet Socialist Republics). It was a union of multiple subnational republics, its government and economy were highly centralized. The Soviet Union was a one-party state, governed by the Communist Party with Moscow as its capital.
It is the most commonly used country for the antagonists in Bond stories due to most being written in the 1950's, the height of the Cold War.
The Soviet Union is present from the beginning of the Bond canon, being the controllers of Bond's initial foes SMERSH.
Bond only travels to the Soviet Union once. At the end of You Only Live Twice an amnesia ridden Bond travels to Vladivostok in search of clues to regain his memory. At the beginning of The Man with the Golden Gun, Bond makes it back to London after staying in the Soviet Union for some months. He was taken by "Colonel Boris" of the KGB to Leningrad where he was brainwashed into attempting to assassinate M.