Throughout his meetings with the two western leaders, President Franklin Roosevelt and British Prime Minister Winston Churchill, Stalin pushed for military and economic assistance for the Soviet Union while demanding that they recognize Soviet dominance of Eastern and Central Europe. At the Tehran Conference in 1943, and again at Yalta in February of 1945, he pushed them to allow what amounted to a "Soviet bloc" extending from the Baltic States across Poland and into Germany, and then down through Southern Europe into Yugoslavia.
Stalin had begun this Soviet-ization with the murder of 15,000 Polish army officers in the Katyn Forest in April 1940, and while the German invasion had interrupted the effort, he was eager to clamp down again. Roosevelt and Churchill, unwilling to antagonize their ally, essentially gave in to his demands--although given the circumstances, they had little choice. (Neither knew that Stalin's spies were at work in the United States, and had already sent information on the atomic bomb project back to Russia, where Soviet scientists were hard at work on their own nuclear weapon.) Churchill appreciated the sacrifices the Russians had made during the war, and wanted to be conciliatory toward them, and Roosevelt seems to have decided that he could "manage" Stalin. But the West would soon have cause to regret these attitudes.