There are many parallels between Hitler and Stalin. On the personal level, they both liked to conduct all-night meetings. On a more critical level, the purge of the Red Army command that Stalin carried out before World War II in order to consolidate his hold on power left the Red Army ill equipped to handle Hitler’s invasion.
While Hitler didn’t order his army command, aside from those who tried to assassinate him, executed, he regularly demoted generals (only later to often promote them again). As a self-styled military strategist prone to overruling his command, he regularly put the Wehrmacht in compromising positions. But while Stalin was able to overcome handicapping Russia at the start of the war, Hitler was not as lucky with another of his purges. In his classic The Second World War (Penguin 1990), which I just finished reading, John Keegan, the eminent ― and eminently readable ― British military historian who died last year, explains.