After World War II, U.S.S.R. did not pull out of Iran, in order to continue getting oil concessions to match those being given to the British. They also began supporting a revolutionary movement in the north (Azerbaijan Province) of Iran, which bordered Russia. The Soviets moved armored units to that border and it was doubtful that they would remove troops from Iran by March 2, 1946, as was agreed at the London Conference of the Council of Foreign Ministers.
President Harry S Truman quickly arranged a meeting with Soviet Ambassador Andrei Andreievich Gromyko and delivers an ultimatum for the U.S.S.R. to remove troops in 48 hours or, "We're going to drop it (an atomic bomb) on you." Needless to say, the troops were removed. It is of note that the soviet rejection of the Baruch Plan for the control of atomic weapons was directly effected by their experience with America’s idea of "atomic control" in the crisis over Azerbaijan.