A battle in which Soviet city marked the beginning of Germany's collapse in the Soviet Union?

Stalingrad.
The problem was, Germany was trying to fight a war on two fronts, which is never a good idea, no matter how powerful of a military you have.
The Battle of Stalingrad in 1942 resulted in a Soviet victory and the complete decimation of the Germany 6th Army. The Soviet forces had heavy losses, but did manage to push back Axis forces from the ground they took over that summer.