Why did stalin join the allied forces in ww2?

The first years of the war the Soviet Union (Stalin) and Nazi Germany (Hitler) had a "no-attacking" agreement and in a joint attack (from the east and the west) ravaged Poland.
After that Hitler (despite the ongoing battles/fronts in the west) attacks the Soviet Union.
At this point there are two big fronts of war and one smaller. Now the allied forces have taken Afrika and want to advance on Italy and France but they need someone to put pressure on the eastern front. Stalin on the other hand wants to liberate all of the soviet lands and form one big Russia. (later known as USSR/CCCP) The leaders of 3 great forces meet and devise a plan to attack at the same time from all sides so they can split the remaining nazi forces. And they do so.

Hope you enjoyed this short history lesson. ;)