Even before signing a devil’s pact with the Germans
Stalin’s paranoia caused him to execute many thousands
of his high level military officers throughout the 1930s.
That set the defense of the USSR back immeasurably.
The Molotov-Ribbentrop pact did not come out of the blue. Yes, lots of soviet apologists and supporters in the West were taken by surprise, but as a matter of fact the collaboration between Stalin and Hitler had grown since the early 1930’s. It was a clandestine effort carried out against a background of massive propaganda. Initially the two dictators used each other to eliminate perceived and real internal rivals and enemies. Of course Hitler would never have been able to start his expansion to the east without a tacit approval by Stalin.
Stalin’s machiavellian policies made the world war almost inevitable, and the russian losses should be seen in that light.
After the war most of this history has been hidden, and initially were only retained in autobiographies of people who had been involved in communist movements (eg Arthur Koestler, Mannes Sperber) and of course in the contemporary statements by defectors like Walter Krivitsky.
A good history regarding this time period (1930 -1941) is Double Lives by Stephen Koch. See also Chekisty by John Dziak. And for those who are fond of historical novels Alan Furst’s Dark Star is must read.
To me, it is astonishing. You hear Russians today, actually hear it, that Stalin’s crimes against his own people are exaggerated and overblown.
I can only shake my head.