I cannot agree with the above post that the Soviets were anxious to appease Germany. Appeasement in any form was not the philosophy of Joseph Stalin. Stalin and Hitler both hoped to dominate Europe: Hitler to find Lebensraum for the German people, Stalin as part of the world wide communist revolution. Any cooperation between the two was part of a grander plan to literally divide Europe between them, beginning with Poland.
Hitler's invasion of the Soviet Union was intended to seize oil fields in the Ukraine and the wheat bearing areas to fuel and feed Germany'w war effort. His plan was enhanced by his personal belief that the German people were superior to the Slavs whom he considered only slightly less inferior than Jews, and that the Soviet system itself was corrupt and would collapse under its own weight. When he discussed his plans with his generals (who attempted to dissuade him) he told them
You only have to kick in the door and the whole rotten structure will come crashing down.
When told that the invasion of Russia had been Napoleon's undoing, Hitler confidently assured his generals that he had learned from Napoleon's mistakes, and would not repeat them. Stalin had previously been confident that Hitler would not invade; so Operation Barbarossa came as a suprise to both Stalin and the Soviet military.
Winston Churchill had once stated that the only thing than fighting with allies was fighting without them. His remarks were cogent in this instance: The Soviets were only limited allies, basically fighting their own war for their own reasons.