A very small part, the whole of the USSR was responsible; especially due to the sacrifices and hard times the folks living there went through just to be the largest force in defeating european fascism. Every socialist who fought in Europe in Europe was responsibly for defeating fascism. In reality Stalin was not the sole person responsible for every decision and event in the USSR, he was only one representative part of a more cohesive whole, but reminding liberals that in their eyes if they childishly view Stalin as the sole person responsible for every decision in the Soviet Union during his tenure as General Secretary then he's also the man who had the guts to finally put the end to the holocaust and give Nazis the most impossible conflict. Socialists saved the world from German Fascism, but Stalin represents the most successful movement that was able to do so.

To be fair, after his significant blunder of underestimating how quickly the Nazis would attack, he turned out to be an excellent strategist in the war, and despite being inadequately prepared for the attack even the state they were in was significantly due to Stalin (not alone, but included) having years before the Pact identified Hitler and Nazism as a mortal threat that the Red Army needed to prepare for, and pursued various measures to that end.