The Great Purge was denounced by Soviet leader Nikita Khrushchev following Stalin's death. In his secret speech to the 20th CPSU congress in February 1956 (which was made public a month later), Khrushchev referred to the purges as an "abuse of power" by Stalin which resulted in enormous harm to the country. In the same speech, he recognized that many of the victims were innocent and were convicted on the basis of false confessions extracted by torture. To take that position was politically useful to Khrushchev, as he was at that time engaged in a power struggle with rivals who had been associated with the Purge, the so-called Anti-Party Group. The new line on the Great Purges undermined their power, and helped propel him to the Chairmanship of the Council of Ministers. Starting from 1954, some of the convictions were overturned. Mikhail Tukhachevsky and other generals convicted in the Trial of Red Army Generals were declared innocent ("rehabilitated") in 1957. The former Politburo members Yan Rudzutak and Stanislav Kosior and many lower-level victims were also declared innocent in the 1950s. Nikolai Bukharin and others convicted in the Moscow Trials were not rehabilitated until as late as 1988. Leon Trotsky, considered a major player in the Russian Revolution and a major contributor to Marxist theory, was never rehabilitated by the USSR. The book "Rehabilitation: The Political Processes of the 1930s–50s" (Реабилитация. Политические процессы 30-50-х годов) (1991) contains a large amount of newly presented original archive material: transcripts of interrogations, letters of convicts, and photos. The material demonstrates in detail how numerous show trials were fabricated.