Germany and Japan seriously underestimated Americans ability to produce war supplies.
Step-by-step explanation:
Germany and Japan both were superior in troops and weapons production. They were technologically upgraded and master mind in planning the supply procurement time to time. They underestimated Americans ability to produce war supplies.
In world war I air, water and land all three space were occupied for fighting. Defense, navy and air force were in charge of setting the required weapons and troops. America was good enough to produce naval weapons. German and Japan commanders and soldiers were mainly underestimate the ability of America.