During world war 2, the battle that turned the tide of war against germany on the eastern front was the

This would be the Battle of Stalingrad. It was when the Nazis fought the Soviet Union for the city of Stalingrad, which is now Volgograd. 
The reason this was a turning point in the war is because up until this point, Germany seemed unstoppable. This is also considered one of the bloodiest battles in the history of warfare. When the Germans attacked, they pushed the Soviet forces farther into Russia, thinking they had the advantage. However, come November, the Soviet troops surround and cut off the Nazi troop. The battle didn't end quickly, as it took until nearly February for the German's ammunition and food supply to completely drain. However, in the end, they did surrender. 

Hope this helps!