After the end of WWII, American intelligence efforts turned to the Soviet Union. This began the Cold War, which encompasses espionage, political, and military conflicts between the United States and the Soviet Union.
The Soviet Intelligence Agency KGB was successful in deeply penetrating sensitive areas of the US government. This was usually done with the cooperation of US citizens, who found philosophical objection to the possibility of an American monopoly on nuclear technology, as threat to peace and freedom.
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