The Great Purge is a period in the 1930s of mass repression in the Soviet Union during which the Communist Party leadership under Joseph Stalin used execution and mass imprisonment to destroy any potential political opposition.
Anyone perceived as a potential threat to the regime's authority -- including some of its strongest political supporters, and most senior army officers -- were systematically identified and either executed, incarcerated in the Gulag prison system, or sent into forced labour or internal exile in Siberia and other remote regions.
The most intense period of the Purge was from 1936 to 1938, while Nikolai Ivanovich Yezhov was head of the ministry of internal affairs NKVD. In Moscow, several show trials were held, to convince domestic and foreign opinion of the existence of a vast anti-Soviet conspiracy and to serve as examples for the trials that local courts were expected to carry out elsewhere in the country.
Almost all of the Bolsheviks who had played prominent roles during the 1917 Russian Revolution, or in Lenin's Soviet government afterwards, were executed or exiled during this period. Leon Trotsky went into exile in Mexico, but was murdered by a Soviet agent in 1940. Of the senior revolutionary Bolsheviks, only Molotov and Stalin himself survived the Great Purges unscathed.
There were four key trials from 1936 to 1938: the Trial of the Sixteen (August 1936); the Trial of the Seventeen (January 1937); the trial of Red Army generals, including Marshal Tukhachevsky (June 1937); and finally the Trial of the Twenty One in March 1938.
Millions were arrested and sent off to prison or labour camps. By summer 1938, everyone in power realised that the purges had gone too far, and Yezhov was demoted to People's Commissar of Water Transport on August 21. Lavrenty Beria then became head of the NKVD. This signaled the end of the Great Purge, although the practice of mass arrest and exile was continued until Stalin's death in 1953.