It's not really clear that Stalin's great purge caused the Soviet defeats in 1941

One of the points that Western historians harp upon regarding WWII, is that the massive losses suffered by the Soviet Union in 1941 were directly attributable to the Great Purge of the Soviet military hierarchy by Josef Stalin in the late 1930's. These historians contend that with a more intact senior officer corp, the USSR wouldn't have been as poorly equipped to defend effectively against Hitler's Operation Barbarossa, and that the losses of manpower and territory would have been far more modest. Now, this is of course arguable, but, is it really provable? After all, no one contends that the reason that Pearl Harbor was such a disaster for the US, was that FDR had purged the US military. Some suggest that they should have been better prepared, there are even some conspiracy theorists who argue that FDR deliberately sacrificed them to provide a military justification to enter the war, but, no one really takes them very seriously. The fact is, at the beginning of WWII, military technology favored the attacker. Tanks and fighter-bombers had shifted the advantage from defense -- which gave entrenched heavy machine guns the advantage during WWI -- to the offense. Inevitably, any moderately well prepared attacker was going to be able to advance quickly, because of this basic change in military technology. So, it is no miracle that the attackers in WWII -- Germany and Japan -- did very well at first. It may be tempting to attribute this to some fundamental error on the part of the defenders, but, this is probably not the case. In the case of the Soviet Union and Josef Stalin, this temptation is aggravated by the deep hatred and distrust that Western historians have for Stalin and Communism in general. So, the Soviet disasters at the start of WWII are inevitably and consistently attributed by Western historians to the Soviet purges in the 1930's. This is much less the case with Russian historians who, these days, are by no means fans of communism. They see these military disasters as the result of the effectiveness of the Nazi military, and the advantages accruing to the attacker. They are probably correct. Stalin wanted to ensure absolute control over the Soviet military, and absolute loyalty. These may have been advantages that allowed the USSR to win the war. Stalin may have been right.