During the years 1937-1938, the Soviet government engaged in a series of campaigns against perceived enemies of the state that resulted in hundreds of thousands of executions. Known as the Great Terror or Great Purges this blood letting constituted the majority of death sentences under Soviet rule. Between 1921 to 1953, a total of 799,455 people tried by Soviet police organs (Cheka, GPU, OGPU, NKVD and MVD) received death sentences. The NKVD issued a full 681,692 of these sentences in 1937 and 1938. Over a third of these executions came from the national operations aimed at eliminating agents of specified foreign powers. The "German Operation", "Polish Operation", "Finnish Operation", "Latvian Operation" and similar campaigns accounted for 247,157 death sentences. These executions fell heavily upon Soviet citizens belonging to extra territorial nationalities. These groups constituted only 1.7% of the population of the USSR, but accounted for over a quarter of all arrests and executions during the Great Terror.
The figure for executions is of course incomplete. It only includes executions approved by NKVD boards. It does not include NKVD executions by decree such as those at Katyn, Rivne, Lviv, Lutsk, Tallinn, Tartu, Kharkhiv and other areas in 1940 and 1941. Other Soviet records for instance register the execution of 21,257 Polish prisoners in accordance with a 5 March 1940 decree signed by Beria. The German military later uncovered the bodies of the 4,421 of these detainees shot at Katyn. These executions are not part of the recorded total cited in the paragraph above. The figure also does not include death sentences handed down by regular courts rather than the NKVD. Nor does it include the mass shootings of Soviet soldiers accused of desertion in 1941. In total the Stalin regime probably executed close to one million people, the vast majority of them for alleged political crimes.
Figures cited are found in the works below.
J. Otto Pohl, The Stalinist Penal System (Jefferson, NC: McFarland, 1997), pp. 7-9.
Terry Martin, An Affirmative Action Empire: Nations and Nationalism in the Soviet Union, 1923-1939 (London: Cornell University Press, 2001), pp.338-339.
G.C. Malcher, Blank Pages: Soviet Genocide Against the Polish People (Surrey: Pryford Press, 1991), p. 168.