The Soviets defeated Hitler when the German troops couldn’t reach Moscow. They were not prepared for a winter war in Russia, they had no Winter clothes and the soldiers were exhausted from months of battling. In August 1942, The German army has few reserves of food and water when they tried to reached Stalingrad.
The battles were ferocious. Germans put their best troops into the city, but the mistake was that they left the rear to Romanian and Italian forces that were not as well prepared to guard the post.
Russian army broke through that part and surrounded those troops. Short of food and ammunition, the Germans. On 31 January 1943, the German army surrendered.