The German Nazi era is still open to discussion regarding the military strategies they used which finally took them to be defeated by the allied forces, specially because they were highly pesonalized and depended on the will of the leader of the state, so, there are many reasons which can explain why the German Army fell.
One of the biggest reasons is that the war had two open fronts, both equelly challenging for any army: the east front with Russia, and the West front, fighting against the western powers: U.S., Britain, France, Canada and Autralia. The allied powers outnumbered the German Army, and it was really difficult to produce and replace the warfare which was destroyed to continue the war; the weather in the east front did not help the victories that technically had to be quick and lethal, and in that order, the German Army had to withdraw with terrible consequences, as their morale was quicly undermined.
Althouhgh Germany expected the allied forces to invade France, they could not forecast that this invasion was going to be that massive, therefore, even when they were prepared their efforts were not enough and they could not block the allied forces to enter Europe through France, and this was a situation very difficult to reverse, prompting the fall of the German forces.
As a conclusion, the difficulties of fighting a war in two different fronts at the same time, had two direct consequences: first, that supplies and replacement of warfare, such as tanks, airplanes and others were not possible to produce as the war demanded, and secondly, this helped the allied forces to easily outnumber the German Army although it might have been superior in several aspects, and finally, weather in the east front, which contributed to the defeat in Russian territory.