Germany and Japan seriously underestimated Americans ability to?

They underestimate America ability to produce weapon.

Explanation:

This underestimation occurs during the WW 11 where the country of Germany and Japan look down of America ability to produce weapons to fight during the war. They underestimate America ability to produce war armory like ships, airplane, vehicle in a fast moment to fight against them during the war.

They also underestimate them being unable to increase the number of their Merchant Marine of the U. S navy to provide assistance to the U. S and to some of her allies.