From Stalin to Kruschev
Stalin's rule between 1940 through 1953 was notable for the USSR's role in the Second World War, from which it emerged as one of the world's two superpowers and consolidation of the Soviet Empire in Eastern and Central Europe (countries were Albania, Bulgaria, Czechoslovakia, GDR, Hungary, Poland, Romania, and Yugoslavia). The entire system was directly controlled from Moscow (with the exception of Yugoslavia under Tito, who had made his own revolution).
One of Stalin's last acts before his death in March 1953 was the orchestration of the "Doctor's Plot". A number of Kremlin physicians, most of them with Jewish names, were arrested for the murder of Zhdanov (one of Stalin's chief lieutenants). It gave Stalin the pretext for a widening purge of "traitors" working for American imperialism, Zionism, and the destruction of communism. Stalin died in the midst of this situation and his hiers declared the Doctor's Plot a police fabrication.
At Stalin's funeral, the crowds were such that several people were crushed to death.
Krushchev and De-Stalinization
When Krushchev took power, his principal concern were the problems engendered by 24 years of Stalinism. The whole movement of "Reform Communism" instituted by Krushchev was based on dismantling the machinery of Stalin's personal dictatorship. After Stalin's death, ruling power passed at first to another triumvirate- Molotov, Beria, and Malenkov- all Stalin loyalists. Krushchev was initially underestimated by his colleagues and assigned as First Secretary of the Party, which by then had become a secondary position. Krushchev first eliminated Beria (the most heinous of the Stalin cronies) by organizing other Presidium members (new name for Politburo) for Beria's overthrow. Beria was arrested at a Presidium meeting, was tried in secret, and executed several months later. His execution was the last for a major political figure. By 1955, Krushchev had also forced Malenkov's resignation. He still hoped to get rid of rivals Molotov and Kaganovich and consolidate his leadership. He believed that by attacking Stalin publicly, both of these men would be discredited and thereby politically neutralized.
The Secret Speech
Khruschev made his attack before the 20th Party Congress in 1956 in what came to be known as the "secret speech." In it, Khruschev denounced only some of Stalin's crimes, and only those committed against the Party and only after 1936. What was significant, however, was that the head of the Party and of Soviet government had publicly admitted that criminal activity was at the heart of Soviet power. While it is undoubtedly the most important thing Khrushchev ever did, the speech had several consequences, not all of them good. Most significantly, it began the process of dismantling the myth that had become the foundation of Soviet communism.