Joseph Stalin wanted to get rid of the threats to his power base, but theses threats were only in his mind due to his paranoia. This paranoia led him to the cost of 13 million deaths. He had millions of people arrested on trumped up charges, and made so called show trials were they were forced to admit their guilt, then they were executed or sent to Siberian labor camps. Between 1936 and 1938, thirty six percent of communists membership were purged. No one dared to protest for fear or retribution. Stalin turned his gaze to the armed forces, for he wanted every aspect of life under his control. They had escaped prosecution, and were at the time the most modern in the world. Three out of five marshals and 14 out of sixteen army commanders were charged for being agents of foreign powers, and were secretly tried and executed. 
His will worked its way down to Junior Officers. Almost half of the officer core was purged. The Red Army’s armored formation was broken up, incompetent officers were given promotions. Stalin realized his great purge had been a big mistake. When they attacked Finland at the end of November 1940, his forces suffered blood reverse. Strain was then forced to institute whole whole sale reorganization and reformatzion of Soviet armed forces. But forms had not been completed when Germany attacked in June of 1941. Soviets poor performance cost the lives of hundreds of thousands lives, and enabled Hitlers army to advance nearly to the gates of Moscow before they were stomped by the Russian Winter.