In the early stages of World War II , both Nazi Germany and the USSR invaded and occupied the countries of Eastern Europe after the Molotov – Ribbentrop Pact . Germany then turned against and invaded the USSR : the battles of this Eastern Front were the largest in history . The USSR joined with the Allies and in conferences at Tehran and Yalta , the Allies agreed that Central and Eastern Europe would be in the " Soviet sphere of political influence . " . The USSR fought the Germans to a standstill and finally began driving them back , reaching Berlin before the end of the war . Nazi ideology was violently anti-communist , and the Nazis brutally suppressed communist movements in the countries it occupied . Communists played a large part in the resistance to the Nazis in these countries . As the Soviets forced the Germans back , they assumed temporary control of these devastated areas
Yalta
Yalta Conference
Revolutions of 1989