The Nazi Germany has reached an agreement of mutual support of the Soviet Union and has divided the world. As far as I can remember, Estonia had to be given to the Soviets. Can't recall atm.
Then, the Soviets occupied half of Poland, which had to be given to them according to the agreement and the other half was owned by the Third Reich, and the German forces attacked the USSR in 1941 (while the war began in 1939, so the Russians joined two years later after the beginning), as Hitler suspected the USSR of plotting an attack against Germany.
Now, the casualties.
wiki.answers.com has written
Estimates range from about 50-70 million killed in World War 2. The Wikipedia article "World War II Casualties" favours the relatively high figure of 72 million. Of these, 61 million were on the Allied side and 11 million were on the Axis side. The article gives a figure of 23 million dead for the Soviet Union and 20 million for China. These figures of course include civilian dead. (Note that the figure for China was recently revised sharply upwards from earlier estimates of about 11 million).
For Germany the overall total is given as just under 7.4 million.
For Germany the overall total is given as just under 7.4 million.
So basically, there were more British, American and other people dead than Russians.
tl;dr - Russians were allied with Germans in the beginning and there were more Europeans and Americans killed on the Allied side than Russians.