The winter cold.
Hitler thought he would of won the battle by then, so he didn't give his soldiers warm clothes, but the Soviet Union refused to surrender so the German soldiers started to freeze during winter.
The German Army lost the Battle of Stalingrad due to the harsh Russian winter that they were not prepared for. The temperatures dropped and froze their machines since they were not built for such temperatures.
I hope this helps ＼(^^＼)
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