Between 1937 and 1939, Stalin turned his paranoid thoughts towards the Red Army. The army had been built up over the years firstly by Leon Trotsky and then by Tukachevsky. Their numbers had grown to several million. During this time Stalin managed to purge 37,000 members of the Red Army.
The multiple waves of military purges, which began in 1937 and lasted into the opening months of World War II, liquidated most Red Army theoreticians and senior commanders. Inevitably, therefore, their ideas fell into disuse or outright disrepute. The families, the friends, and the colleagues of the condemned either joined them in oblivion or sat with faces frozen in mute resignation, waiting for the summons that could arrive at any moment
Although the senior ranks experienced the most severe losses in terms of percentages (11 of 13 army commanders were shot, as were 57 of the 85 corps commanders and 110 of the 195 division commanders), the numerical bulk of the victims came from subordinates unfortunate enough to be on the wrong staff or performing the wrong mission. Estimates of the total losses created by this mass bloodletting range from 15,000 to 30,000 officers, depending upon the dates used and the figures available. And, most of the 1,836,000 surviving Red Army prisoners of war liberated from the Axis powers at the end of World War II were sent to the Gulag as "traitors to the motherland."
Stalin wanted a military establishment with which he could be comfortable, a docile instrument of his will that would do his bidding and with which he would not have to negotiate in order to attain his objectives. If he lost some talent in the pruning, it was a small price to pay: Talent was expendable; peace of mind, precious.
Trotsky took the view that the purges were part of Stalin's scheme to ensure the loyalty of the Army chief, Voroshilov: "The military machine is very exacting and voracious and does riot easily endure the limitations imposed upon it by politicians, by civilians. Foreseeing the possibility of conflicts with that powerful machine in the future, Stalin decided to put Voroshilov in his place before he began to get out of hand. Through the OGPU [a former Soviet secret service organization], i.e., through Yezhov, Stalin prepared the extermination of Voroshilov's closest collaborators behind his back and without his knowledge, and at the last moment confronted him with the necessity to choose. Thus trapped by Stalin's apprehensiveness and disloyalty, Voroshilov collaborated in the extermination of the flower of the commanding staff and ever after was doomed to cut a sorry and impotent figure incapable of ever opposing Stalin. Stalin is a past master of the art of tying a man to him not by winning his admiration but by forcing him into complicity in heinous and unforgivable crimes. Such are the bricks of the pyramid of which Stalin is the peak."
Stalin's defeat in Finland in 1940 and his retreat from Hitler's forces in 1941 came from purging his own army. The results of the Stalinist bloodbath showed up in the poor performance of the Red Army in the winter war with Finland (1939-40): well over a million well-armed men were stalled for months before a thinly defended Finnish line, and the Soviet losses were almost unbelievable. This bitter experience did, however, pinpoint some of the Red Army's worst shortcomings and resulted in the replacement of Voroshilov as the defense chief, an event long overdue. Historians are unable to explain why the purge happened, possibly a task for the student of abnormal psychology, not for the historian.