> Economic policy alone would not have resulted in the powerhouse America became

During WW2, the US was the most powerful economy in the world. It fought WW2 in both hemispheres (which required two navies), and supplied all the allies.

Japan was well aware they could never win a sustained war with the US, and hoped that the attack on Pearl Harbor would get them a negotiated settlement. Hitler, in probably the stupidest miscalculation in history, declared war on the US.