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what are the choices?
Frankly, smashing the entire Sixth Army into tiny pieces, halting the attack into the Soviet Union, stunning the world and showing that the "invincible" Germans could be defeated would make it a turning point.
Also, it was winter, an idiotic decision to let the Germans fight it out themselves, and facing three different armies at the exact same time and forgetting that winter = bring winter clothing contributed to the defeat.
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A lot of it was that it was a major industrial city. A large amount of Russian war resources were produced there. Also sense Hitler hated communism and Stalin so much he wanted it as a token per say to prove he was better. 3rd reason was that it was had a river called the Volga which lead directly to Russia. Last reason was that Hitler wasted a lot of resources and soldiers to try to capture it.