There are a number of different ways of looking at this.
For example, we can look at the ultimate cause of the Allied victory, which was America's industrial capacity. The Allies were able to win because the US could put out so much in the way of war materiel. This allowed the Allies to overwhelm the Germans.
We can also say that the Allies won because Russia bled the German army badly while the US and Britain conducted strategic bombing. If the Soviets had not been able to destroy so much of the German war machine, the Allies would not have been able to win. A related argument here would be that the Allies won because Hitler made the mistake of invading the USSR.
Finally, we can answer this by talking about the events that led to Allied win. Looked at like this, we say that the Allies won by having the Soviets fight in the east while the western Allies gradually started to fight in the west. The western Allies defeated the Axis in Africa and then in Italy. Finally, they invaded France on D-Day and drove over land into Germany.