War broke out between Britain, France, and Germany, on 3 September 1939. At the end of August, the Soviet Union and Germany had entered into a pact of non-aggression. At this time, Stalin hoped to stand aside from the conflict in the west, and also to exploit it, expanding Soviet territory and military security at the expense of Polish and Baltic independence. But during the brief period of Soviet-German cooperation, Hitler's long-term perspective for German expansion remained fixed firmly on Soviet soil.

Already dominating half of continental Europe, German forces launched their surprise attack on Russia on 22 June 1941. The greatest land war of all time had begun. It would be fought with tens of millions of soldiers, and hundreds of thousands of aircraft, tanks and guns on each side, along a 2000 kilometre front.

By mid-autumn Kiev was taken, Leningrad besieged, and Moscow directly endangered. But neither Leningrad nor Moscow fell. The battle of Moscow in the autumn and winter of 1941–2 denied Hitler the lightning victory on which his hopes were pinned. Germany had lost the initiative, and now struggled to win it back. In the spring of 1942 the Wehrmacht advanced in a great arc across the south towards Stalingrad and the Caucasian oilfields. The Soviet encirclement and destruction of huge German forces at Stalingrad in the winter of 1942–3 marked the turning point.