Sanctions constrained Japanese munitions production

Would the United States simply have stayed out of the way of Japan's imperial ambitions?

Not safely: American sanctions were already constraining Japanese access to materials needed for weapons and ammunition.

The answer cautions against assuming that American public reluctance for war meant Japanese expansion faced no resistance. Washington was already applying economic sanctions before Pearl Harbor. Scrap-iron restrictions mattered because Japan depended on imported material for ammunition and wider war production. The strategic crisis therefore involved access to resources as well as public opinion and formal declarations of war.