Speaking of Stalingrad, as people were downblog, Brad DeLong has a nice 70 years on retrospective; on the battle and on the Eastern front generally:
The Allies probably would have eventually won World War II even had the Nazis conquered Stalingrad, redistributed their spearhead forces as mobile reserves, repelled the Red Army’s subsequent winter 1942 offensive, and seized the Caucasus oil fields, thus depriving the Red Army of 90% of its motor fuel. But any Allied victory would have required the large-scale use of nuclear weapons, and a death toll in Europe that would most likely have been twice the actual World War II death toll of perhaps 40 million.
May there never be another such battle. May we never need another one.
Any specific claim as to how the war would have gone if the Soviet Union had been knocked out of active participation has to be speculative. However, I only had one relative die as a consequence of World War Two (a maternal great uncle, at Anzio, with the North Staffordshires), and I don't think there's any doubt that I have the Soviets to thank for that, above and beyond the contribution made by other allied nations. Given that five out of every six German combatants killed were killed by the Soviets, it couldn't have been any other way.