In August 1939, Josef Stalin freed Hitler from his fear of the possibility of a war on two fronts by signing a pact of non-aggression with Germany. As a result, a month later, Hitler invaded Poland and precipitated the Second World War.
But, the security offered by the Brest-Litovsk treaty was short-lived. Hitler's rapid victories in the west made Stalin increasingly nervous. Despite repeated attempts to maintain the terms of the pact, in June 1941 Germany invaded the USSR.
At first, Stalin suffered a nervous breakdown and was unable to command the Red Army or delegate effective control. Furthermore, his purges of army officers in the 1930s had crippled the Soviet military machine.
However, within weeks Stalin had recovered. Although German troops had made huge advances into the heartlands of the USSR and the rest of the government had been evacuated from Moscow, Stalin remained in the Kremlin and begun to take control of the country's ailing military infrastructure.
Industrial plants were moved from European USSR to the east, competent military commanders were allowed to take control of important strategic positions and Soviet ports accepted increasing quantities of lend-lease products from the Western powers. At the same time, Stalin helped to forge a formidable nationalist morale out of the traditional religious and patriotic sentiments of the Russian people.
In December 1941, Russian forces pushed the Germans from their positions around Moscow. Over the next three years counter-offensives at Stalingrad and Kursk further diminished the power of Hitler's eastern army. By late 1944 Soviet forces had entered Eastern Europe and in April 1945 they started the final attack on Berlin.
As Russia's effort turned the war decisively against Germany, Stalin started to jostle for post-war position with the other allied countries. At the Tehran Conference in 1943 and the Yalta and Potsdam Conferences two years later Russian sacrifice and Stalin's imposing diplomatic manner forced Franklin Delano Roosevelt and Winston Churchill to accept Soviet influence in Eastern Europe.