The Battle of Stalingrad

Theo Wilson time-travels to the Battle of Stalingrad in 1942 where the underdog Russian Army is about to pull off a victory that will turn the tide of the war in favor of Allied forces. Bombs explode, concrete rains down and soldiers' limbs freeze in subzero temperatures as the bloodiest battle in history wages on--and Wilson wants to know, how did the rag tag Russians emerge with a decisive win?