Stalin and Pol Pot were leaders in the Communist Party. The Communist Party banned religion because it was considered opium for the masses.
The USSR was the first country to completely abolish religion, taught atheisim in schools, and persecuted believers of any faith. Karl Marx said "The first requisite for the happiness of the people is the abolition of religion."
One of the first things the Khmer Rouge did when they took over Cambodia was to ban all capitalism, western influence, and religion. He never made his personal beliefs on God known that I know of, so it's impossible to say if he was an atheist or not. However, since he was a communist, it's pretty much a given.
Hitler wasn't an atheist any more than he was a Christian. He was a Ayran/Hitlerist. In other words, he thought he was a god.
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"Hitler was a neo-pagan terrorist whose conscience was not informed by Christianity, but by pseudo-scientific racist philosophies. Hitler hated the Catholic Church, made plans to kill the Pope, authorized the murder of thousands of priests and nuns, and did everything he could to suppress the influence of the Church. In 1933, Hitler said, 'It is through the peasantry that we shall really be able to destroy Christianity because there is in them a true religion rooted in nature and blood.'" The Catholic League also quoted Hitler, in a 4/23/99 Op-Ed ad in the New York Times, as saying, "Antiquity was better than modern times, because it didn't know Christianity and syphilis."
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