Joseph Stalin was the second Premier of the Soviet Union, just right after Vladimir Lenin died of a heart stroke.
Joseph Stalin developed his own version of communism - Stalinism, slightly making it different from Lenin's original plans. Stalinism made Soviet Union more authoritarian, but still having a party, and dictatorship of Proletariat. Stalin truly believed in communism, and that's why he derived some points from his opponent - Trotsky, who was assassinated in Mexico where he flew after The Great Purge begin. Some of Trotsky's influence was seen after the end of WW2, where both USSR and USA tried to gather as many puppet states as possible.
In the nutshell, Joseph Stalin was a dictator who changed Lenin's original plans by getting control over USSR. He was a role figure in defeating Nazi Germany, and pupetting Eastern European nations such as Poland after the war. He established Stalinism, which moved further from Leninism and Marxism who were located on the far-left, but still had a Communist Party lead the nation since he did believe it was the optimal pick for Soviet Union.
