I think that one reason of the victory against the axis power in Europe is the fact that United States entered when the war had already begun. It had a fresh army, well trained and with a better resources that was left in Europe. The continental part of Europe was completly under the axis control and Great Britain stood as the only oponent. Some of the government people of that country was in favor of surrender to Hitler to save the life of their countrymen. But after the attack on Pearl Harbour, U.S. got an excuse to entered the war and change the tide. By the time of the landing in Normandy Germany´s army had occupied a lot of terrain and could not defend it well because the army was prepared to conquest not to maintain the territory by the tactic called "Bitzkrieg".
 Another reason for the victory I think is the fact that Hitler did not believe that United States was going to entered the war and put his best resources on the front against the Soviet Union and its red army. Althought this the red army not only defend the terrain with oil that the Germans needed for holding its battallions, it also defeat and make them give terrain becoming the first army to entered Berlin.
 So summon it up a little I believe that two main reasons for the allies victory over the axis powers were. The entrance of the United States army with his vast power and the oposition that the Red Army gave to the Germans not letting them achieve the oil that they needed.
 I hope that the answer is correct and help you.
