The mystery of American victory in World War II was mainly quantity and quality. Bountiful measures of weapons and hardware that overpowered and outmatched the Axis arms stockpile. As well as empowered Lend-Lease partners like Britain and Russia to do likewise.
Not that each U.S. weapon was awesome. For example, the pervasive M-4 Sherman tank was ample yet fair. Early U.S. warriors like the P-40 and P-39 were nothing to gloat about. (with the exception of in the hands of the Flying Tigers). While U.S. submarine torpedoes had a negative behavior pattern of holding off on detonating until late 1943.