Julius Robert Oppenheimer was the scientific director of the Manhattan Project and the first head of the U.S.'s Los Alamos National Laboratory. He oversaw the creation of the fission bomb for its use in World War II, and popularized the Bhagvad-Gita quote "I am become death, the destroyer of worlds" upon the successful test of the bomb in New Mexico. The project led to the construction of the bombs which were detonated over the Japanese cities of Hiroshima and Nagasaki.
Though an undisputed genius, Oppenheimer was a bit lacking in social skills, and therefore was an easy target for political abuse, something which cost him his career at the hands of the better-connected Edward Teller (a true mad scientist if there ever was one) when Oppenheimer opposed the development of the hydrogen bomb (Teller's pet project, and, ironically, one Teller needed to borrow designs from others to develop). He was a communist, although he identified himself as a "fellow traveler" and not a member of the official party; one major point of difference was that he declined to follow any orders from Moscow. This being the McCarthy era, it was very easy to turn this against him and claim that he was a Soviet spy; this eventually led to the revocation of his security clearance. To this day, some doctrinaire conservatives consider Oppenheimer to have been a Soviet spy.
After the collapse of the Soviet Union in the 1990s, the retired head of Red Army counterintelligence (SMERSH) claimed that Oppenheimer had been suckered, partially through exploitation of his fear of nuclear war and his Jewish identity, into cooperating with Soviet intelligence.