Final answer:
The Soviet Union under Stalin played a crucial role in WWII, bearing the brunt of the German attack with over 25 million Soviets dying and establishing a communist bloc in Eastern Europe post-war to serve as a buffer against the West.
Step-by-step explanation:
Under the rule of Joseph Stalin, the Soviet Union's participation in World War II played a pivotal role in the defeat of Nazi Germany. The USSR under Stalin was transformed into a modern state by the eve of the war, and its military efforts on the eastern front were vital to breaking and driving back the German war machine. The Soviet Union bore an immense human cost, with over 25 million Soviets, both soldiers and civilians, losing their lives.
Following the war, Stalin's power in the USSR was unshakable, as he enjoyed a period of popularity for the role he played as a protective leader during the conflict. Furthermore, Stalin ordered the establishment of communist governments in Eastern European nations such as Poland, Czechoslovakia, Hungary, and Romania, which formed a buffer zone against potential attacks from the West. Despite the promise of democratic elections, these nations ended up with one-party communist states that mirrored the USSR's model.
In terms of foreign policy, Stalin enforced Soviet influence on Eastern Europe and maintained territories acquired during initial collaborations with Nazi Germany. Stalin's military victory in what the Soviets called the Great Patriotic War solidified his leadership and extended the Soviet sphere of influence over Eastern Europe. Despite the heavy cost of the war and initial intentions to possibly step down post-war, the post-war geopolitical landscape and the Cold War pressures solidified Stalin's rule until his death in 1953.