The Eastern Front
After the defeat at Stalingrad in february 1943, the German army was retreating.
By this time the Soviet factories were geared to maximum production - they were able to replace any tank or aircraft that was lost in battle.
The Germans, on the other hand, were not able to replace lost tanks and aircraft quickly enough. German production could not increase because the Americans and British were bombing German factories and also rail links between Germany and the USSR.
By august 1943 the Red Army had four times as many tanks as the Germans at the eastern front.
Because the Soviet Union was now an ally with the British and Americans, Stalin urged his new friends to come to his aid by attacking Hitler from the west, so Germany would face a two-front war.
Churchill and the American president Roosevelt promised to open a "Second Front" in the west soon, but they needed much time to prepare this invasion.
Meanwhile, the Red Army marched westward, driving the Germans back.