Why were the allies able to win world war 2?

Answer 1

From the beginning, the Allies had an overwhelming superiority in resources which the Axis could not match. Disorganization, overconfidence, lack of preparation, and outdated military thinking among the Allies allowed the Axis to make such remarkable gains in the first 3-4 years of the war, but eventually the Allies pulled themselves together and used their immense recources to crush the Axis. It could possibly have been done a year or two earlier, but the Allies argued among themselves and did not always use their strength to best advantage.

Answer 2

Italy and Germany ran out of resources, especially trained soldiers.

Japan was doing same but refused to surrender until the 2 fission bombs convinced the emperor that surrender was the only semi-honorable way out (the would be no honor in just having his country erased by these new bombs that would not even give the civilians the right to fight honorably against the "enemy").