Please note that different people might attribute the Allied victory in WWII to different factors. I suggest that you consult your text and/or class notes to see if your instructor expects a specific answer here. My own view is that there are at least four main reasons why the Allies were able to win WWII. These are 1) the population and industrial power of the US, 2) the geographic size and large population of the USSR, 3) the presence of Britain as an Allied land off the coast of Europe, and 4) the mistakes made by the Axis.
The Axis made many important mistakes that helped the Allies win the war. Hitler made a huge mistake when he launched his invasion of the Soviet Union. One can argue that the Japanese should never have attacked Pearl Harbor, bringing the US into the war. After the Pearl Harbor attack, the US declared war on Japan, but not on Germany. Hitler then made another mistake by declaring war on the US, making it certain that the US would start to fight in Europe. All of these mistakes helped the Allies win the war.
When Hitler invaded the USSR, it was a mistake because of the huge expanse of Soviet land and the country’s large population. The Soviets were able to trade space for time, preventing the Germans from defeating them before winter came. They were also able to throw huge numbers of soldiers at the Germans, making up for their technological and tactical inferiority with numbers. If it had not been for the size and population of the Soviet Union, the Allies would have been much weaker.
The United States also had a large population that it could throw into the war. Perhaps more importantly, it had a tremendous industrial base. It could pump out massive amounts of weapons and other materiel that it could use for its own military and that it could share with its allies. The US’s industrial base (and invulnerability to attack) also allowed it to create the atomic bombs which ended the war in the Pacific.
Finally, the Allies won because Britain was, in essence, an unsinkable aircraft carrier and supply depot off the coast of Europe. (We might say that another Axis mistake was Hitler’s failure to invade England.) Once the US entered the war, the Allies could build up a huge force in Britain, waiting until the time was right to invade Europe and eventually destroy Germany.
While other people can cite other factors, these are the four factors that, in my mind, are most responsible for the Allied victory in WWII.