Biggest turning point of the war:
Battle for Moscow, winter of '41

This was supposed to have been the culmination of operation Barbarossa and spelled the end of the line for the USSR.
True, it has been written that the fall of Moscow would have meant niether the end of the Russian war effort nor the failure of Russian morale, but so much of the war between Germany and Russia was permiated by symbolism and ideology that the fall of the Russian capitol would have to have been as severe a blow as the successful defense was a boost to Russian morale...
For the Germans, failing in even this ill-defined objective had to have been crushing.
Regardless of holding on by the narrowest of margins thru to the spring of '42, and launching an offensive almost as spectacular that same year, the failure to capture the Russain capitol must have been the definitive event of the campaign...

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Novus Ordo Seclorum