The great purge, especially of armed forces, seems like the greatest blunder of the early soviet union. It killed countless innocents and culled trained military personnel, because ol' Joe Steel was worried about a coup. Given that it seems to have led to humiliation against Finland and probably led to the red army being woefully unready for Operation Barbarossa, what would happen if Stalin's purges were smaller, focused more on administrators or beurocrats, and the military was, well not untouched, but was basically a routine purge vs anything of note?
I actually think the purge saved the USSR. Germany beat a better military (France) quickly because the will to resist collapsed. France's whole strategy was *always* going to lead to them committing reserves to Belgium. France had one goal alone--avoid WW1 again. It was politically impossible to get the people to seriously fight for years, on French soil, and spend all that money and blood. France wanted to bloody Germany bad in Belgium and then win in a year. When this didn't work out, the memory of the last war led to a general collapse. France could have dragged out fighting in its south and then from its colonies--but they were not going to, this was never a serious option. Vichy France did not arise by mistake in some political vacuum.
Now take Russia. Minus the purges, the persecutions, the sheer human brutality. The USSR would still have less men on their frontiers than the Axis forces come June 22nd. By August 1st, their army would be effectively destroyed. A non-brutalized population would have coup'd Stalin and sued for peace, likely not repeating the mistakes of 1917.
Stalin waited at his dacha expecting to be offed in the first week of the war. Instead, people looked to him for leadership out of fear and being so brutalized knowing nowhere else to turn. And so, the USSR survived 1941 where I think no other power would have. They even lacked guns for many of their men. And bullets. WHy did the people fight? Because they were in a psychological state of submission impossible in Poland, France, or Germany 1941. Compare this to Germany 44-45 where strategic bombing and the holocaust cowed the German populace into a steely submission and brutal fatalism.
And so, the USSR survived simply by outlasting the Germans and letting them over extend their supply lines. In other words, Russia won simply by not giving up. This is despite Russia losing every counter offensive, including after Tyfun, where Russian losses far exceeded Germans. But they fought on, despite millions upon millions of new reserves with zero experience being called up to fight with crap equipment or none at that. Germany could have never anticipated this.
I think in retrospect, Barbarossa was a "good" plan that should have worked. Just like the "Strategic Bombing" should have "forced" German surrender. What WW2 got wrong was that brutalizing populations *does not* win wars, it extends them. If the USSR did not cause a collapse in Manchuko, it's unlikely the a-bombs would have ended the war in Asia. And if this were the case, our whole memory and assessment of deterrence through terror would likely be much different.