On June 22 , 1941 , Germany broke the Molotov – Ribbentrop Pact with Operation Barbarossa , the invasion of the Soviet Union through the territories that the two countries had previously divided . Stalin switched his cooperation from Hitler to Churchill . Britain and the Soviets signed a formal alliance , but the U.S. did not join until after the Attack on Pearl Harbor on December 7 , 1941 . Immediately , there was disagreement between Britain 's ally Poland and the Soviet Union . The British and Poles strongly suspected that when Stalin was cooperating with Hitler , he ordered the execution of about 22,000 Polish officer POWs , at what was later to become known as the Katyn massacre . Still , the Soviets and the Western Allies were forced to cooperate , despite their tensions . The U.S. shipped vast quantities of Lend-Lease material to the Soviets
Katyn massacre
Katyn massacre
Origins of the Cold War