Final answer:
The Soviet Union under Stalin made a significant contribution to the Allied efforts in World War II, eventually helping to defeat Nazi Germany after initially signing a non-aggression pact with them. The USSR suffered immense losses, with over 25 million casualties, largely affecting the outcome on the Eastern Front.
Step-by-step explanation:
The best description of the Soviet Union's participation in World War II under Stalin's command is that the Soviet Union played a significant role in the Allied Powers and was a key contributor to the defeat of Nazi Germany. The Soviet Union did initially align with the Axis Powers through the Molotov-Ribbentrop Pact, effectively dividing Poland and avoiding conflict with Germany.
However, after Germany's invasion of the Soviet Union in June 1941, the USSR became a member of the Allied Powers. Through an immense sacrifice, with more than 25 million Soviet lives lost, the USSR bore the brunt of the German war machine on the Eastern Front. The battles waged by the Soviets, such as the Battle of Stalingrad, were pivotal in turning the tide against Nazi Germany.
Furthermore, the Yalta Conference confirmed the Soviet military occupation of Eastern Europe, as Stalin, Roosevelt, and Churchill agreed upon post-war arrangements. The Soviet Union's involvement and casualties played a significant role in shaping these agreements and the post-war landscape.
Thus, the correct answer to the question is Option C: The Soviet Union played a significant role in the Allied Powers and was a key contributor to the defeat of Nazi Germany.