The US had mass production capacity.
Step-by-step explanation:
Germany, Italy, and Japan were the axis powers at the time of WW II. And the Allied army was America, British, France, and U.S.S.R. The Axis seemed to be the strongest in the first since they had a large territory. But Allies were benefited by having America on their side due to its huge amount of production capacity.
Since they produced weapons and other machines faster when compared to any other countries in the world. Also, it had the advantage of having U.S.S.R. since they had the largest army at the time of WW II.