The economy of the USA was far greater than everyone. It provided the allies with resources. Plus the wartime economy basically crashed unemployment to near zero. This was a very "utopian" time for the USA. It's amazing, the USA was in shambles at the beginning of the war, and in a mere 6 years. Where other nations would be taking decades. To add to that. The USA had factors in which allowed it to be an economic powerhouse. The ability to muster more resources and manpower than its opponents. Think about it, the USA had far more resources than Germany and Japan. Now I would go on, but I'll give it to you in short: They were almost near what they were at the beginning of the roaring 20s.