Final answer:
Under Stalin, the Soviet Union played a pivotal role in WWII, seeing immense casualties but ultimately defeating Nazi Germany. Postwar, Stalin established communist regimes in Eastern Europe, while also moving industries eastward to protect and replenish the Soviet economy.
Step-by-step explanation:
The Soviet Union's participation in World War II under Joseph Stalin's command was crucial in shaping the postwar landscape of Eastern Europe and extending the nation's influence. Stalin's wartime leadership transformed the USSR through immense sacrifice; over 25 million Soviets died in the conflict, which was instrumental in defeating Nazi Germany. In the war's aftermath, Stalin established communist governments in Poland, Czechoslovakia, Hungary, Romania, and Bulgaria, creating a buffer zone to protect against potential attacks from the West. At Yalta, Stalin emphasized the need for Soviet occupation of Eastern Europe to ensure the nation's security. Internally, despite the economy being ravaged by war, the mobilization of state-owned factories and the relocation of industries and population to the east of the Ural Mountains enabled the USSR to restore and even increase its industrial capacity.