Despite the signing of the Molotov-Ribbentrop Pact, relations between Germany and the USSR soon deteriorated. Hitler had always viewed the Soviet Union as a natural target of German expansion. On 22 June 1941, Germany launched an attack on the USSR, drawing it once more into the war.
Operation Barbarossa, as the invasion was named, was to be a simple and quick campaign that would last only a few weeks. The German advance stalled in late autumn, however, short of taking Moscow and Leningrad. Confident of an easy victory, the German military had not planned for a winter campaign. Without proper clothing and equipment for winter warfare, the German troops suffered horribly. German casualties during the first year of the Soviet campaign were 1.3 million, slightly less than half of the original invading force of 3 million men.
During the summer of 1942, Germany launched a second offensive, and made significant gains, but could not completely overcome dogged Soviet resistance. The Battle of Stalingrad was the key engagement in the Soviet campaign. German forces controlled much of the city by September 1942, but the Soviet army and the citizens of Stalingrad were ultimately able to hold off the enemy assault. The defence of Stalingrad was, in fact, the great turning points of the Second World War in Europe. Having halted the Germans at Stalingrad, the Soviets were able to launch a counteroffensive that would eventually take them to Berlin.
The entry of the USSR into the war against Germany also brought about a very significant change in international relations. Traditionally, communist Russia and the Western nations, such as Great Britain, the United States, and Canada, had viewed each other with profound suspicion. The circumstances of war, however, transformed these mortal enemies into valued allies. This spirit of co-operation would disappear in the post-war period, but the significance of the Soviet Union's contribution to the war effort is indisputable. Perhaps no other combatant sacrificed as much through its involvement in the war. An estimated one half of all the people killed during the conflict were Soviet.