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. 

There are two possible ways of interpreting Russian involvement in WWII.  Many people believe Russia entered World War II as a result of a surprise attack by the Germans.  Both countries had a non-aggression pact, but Hitler defied the treaty and launched a surprise attack against the Russian in 1941.  Obviously, once attacked the Russians defended themselves and thus entered the war.

Another school of thought is that Russia was involved with the war as early as 1939.  Russia was desperate to appease Germany and keep themselves free of attack.  They supplied Germany with war materials and secretly agreed to attack Poland and divide it between the two nations.  Additionally, Russia was involved in fighting in the Baltic States, Finland, and Romania.

Again, most historians would say that the invasion of Germany caused Russia to enter WWII, but it could be argued that that they were involved two years earlier.