Final answer:
At the start of WWII, Stalin's Soviet Union invaded Poland, following the secret provisions of the Molotov-Ribbentrop Pact. Subsequently, the Soviet army occupied the Baltic States and attacked Finland in the Winter War, aiming to secure and expand Soviet territory.
Step-by-step explanation:
During the start of World War II, Stalin was invading Poland. This occurred when Nazi Germany and the Soviet Union executed a coordinated invasion of Poland in 1939, a move that spelled the end of the Polish state until the end of WWII. This invasion was in accordance with the secret provisions of the Molotov-Ribbentrop Pact, which had delineated spheres of influence in Eastern Europe between the two powers.
After conquering Eastern Poland, Stalin's forces shifted their focus, moving to occupy the Baltic States and to invade Finland in a conflict known as the Winter War. This phase of Soviet expansion aimed to reestablish the borders of the former Tsarist Russian Empire. The Soviet invasion of Finland met with significant resistance and eventually ended with a peace treaty that ceded some Finnish territories to the Soviet Union.