The right answer is The USSR needed to protect its western border from future attacks.
Besides the Ally victory in the WWII, the USSR lost approximately 20 million Russians. Worried about future attacks, Stalin planned to build a buffer zone of communist friendly states around Russia, in order to avoid future invasion. Something that happened during the WWII and heavily cost millions of Russian lives battling against German invasion.
The conclusion was to take control over Eastern Europe (EE). As the Red Army drove the Nazis back, it occupied large areas of EE, and made sure communist governments took control of power. The Communists controlled the army and chased its opponents. By 1949, all the governments of Eastern Europe, except Yugoslavia, were associated with the Stalinist regime.
