On August 23, 1939, Hitler and Stalin signed a non-aggression pact, known as the Molotov-Ribbentrop Pact. What no one knew at the time was that the pact included a secret agreement through which Hitler and Stalin allied to divide Eastern Europe between them. The first country to be split between the Nazis and the communists was Poland, with the western half going to Hitler and the eastern half to Stalin. The Baltic states and parts of Romania were also divided, as was Finland, paving the way for the Winter War of 1939—the Soviet invasion of Finland.

Between August 1939 and June 1941, Stalin sent Hitler tons of raw materials, essential for the Nazi war effort against France and Britain.

The pact between the Nazis and communists ended on June 22, 1941, when Hitler broke the alliance with Stalin by invading the Soviet Union with nearly 4 million soldiers as part of Operation Barbarossa, the largest military offensive in history.