Final answer:
The Soviet Union gained territory in Eastern Europe, including Poland and the Baltic States, by signing the Nazi-Soviet Pact with Germany.
Step-by-step explanation:
The Soviet Union gained territory in Eastern Europe by signing the Nazi-Soviet Pact with Germany. The pact included secret protocols that divided Poland between Germany and the USSR. Additionally, the Soviet Union also gained control over Finland and the Baltic States of Estonia, Latvia, and Lithuania. This agreement allowed Hitler to focus on conquering the rest of Europe without worrying about a Soviet invasion from the east.