The only answer you can give, the one that makes the most sense, is the first one. Stalin and the Russians were carrying the load of war far beyond what should have been their share. Lenningrad had been under seige for nearly 900 days and Stalingrad was one of the bloodiest battles of the war.
A common explanation is that the Civil War was fought over the moral issue of slavery. In fact, it was the economics of slavery and political control of that system that was central to the conflict.
