You know without someone like Stalin at the helm, the USSR might not have survived WWII. Cold, brutal, but effective :( Clearly the man was a monster -- but I think people forget that the bulk of the fighting in Europe during WWII was on the eastern front, not the western; by the time D day happened, the Germans had mostly been defanged, by the Russians.
Stalin did not save the Soviet Union. The Soviet privates, corporals, sergeants, farmers and workers saved the Soviet Union despite Stalin's stupidity.
Stalin was Hitler's secret ally at first. He actually helped start WW2. Stalin's non-aggression treaty with the Nazis had secret parts where Stalin and Hitler agreed to split Poland and other countries, it defined the respective Nazi and Soviet sphere's of influence of Eastern Europe. So Hitler's invasion of Poland, the event that triggered WW2 in Europe, was in fact done with Stalin's blessings. It was not until Hitler invaded the Soviet Union that Stalin fought against Nazism. If the Soviets had a leader that was a true ally of Britain and France and stood against the Nazi's and promised to defend Poland as Britain and France had done there may have been no WW2. Plus with a different leader the Soviets may have fought more effectively during the early parts of the war. Stalin and Hitler were very much alike in many ways. Besides brutal murderous dictators who killed millions of their own people, they erroneously thought themselves military geniuses and through their idiotic orders destroyed their own armies.
To be honest, Stalin expected that the Nazi's would invade, it just happened years before Stalin expected. He ignored one piece of evidence after another indicating an imminent invasion preferring to cling to his earlier personal expectations. His military incompetence was largely covered up, however since the Soviet collapse Red Army records have become available to western historians and his incompetence has been shown to be far greater than imagined. The west was aware of his various pre-war purges that decimated the leadership of the Red Army, officers being selected for loyalty to Stalin rather than military competence, and the previously mentioned self-denial regarding imminent invasion by the Nazis. Today the world knows, via official Red Army communications and unit records/diaries that some of Stalin's so called great military successes were pure propaganda fantasies. For example official Red Army documents show that Stalin did **not** order the Red Army to fall back to draw the Nazi's deeper into Soviet territory, with longer and more difficult to defend supply lines, and draw them into a trap. He actually ordered units to stand and fight at all costs, to not give any ground, much like Hitler did. Again, the two were so much alike. However the Red Army collapsed and retreated in a disorganized and somewhat panicked manner. The Nazi's going too deep and over extending their lines had more to do with Hitler's idiocy of pushing forward at all costs. Both Hitler and Stalin were incompetent military strategists whose idiotic orders destroyed their own armies. The Soviet Union was only saved because of huge reserves it could pull from Asia, including armies from Siberia that were well equipped for brutal winter warfare.