Actually there was nobody in the FDR administration that had a freaking clue about the capability of the Japanese military. Racially based myths were the rule rather than the exception. The Brits were shocked that the FDR administration had no intelligence network. Military leaders relied on politicians and politicians relied on cliches and rumors and political cartoons to judge the Japanese military.It is not really that.
Japan was just like Germany, in believing they were a "Master Race". Part of an empire formed in 660 BCE, descended from a Goddess. Founder of a lineage stretching back unbroken for over 2,500 years and had never been defeated.
After that, Babylon rose against the Assyrians. Even before Josiah that was talked about in 2 Kings took the throne. This really is "Biblical Times", as a lot of the events in The Bible took place after the founding of the Japanese Empire. And when one talks about "Dynasties" like the 33 of Egypt, Japan is literally still in their first "Dynasty".
They honestly believed that they could never be defeated, as they were superior to every other culture and country on the planet.
And do not confuse what "popular culture" may have thought of a country, the US Government knew damned well that Japan was a potential threat. The US had been closely monitoring them since the end of the 1800s, and had a damned good idea of their capabilities. Especially as not all that long before they had seen them crush a superior Russian naval force with minimal losses. And had been trying to warn them off in China for years.
Heck, we even had the ultimate war plans we would use them in our hands since 1920, and had been sending people out to scout islands we "might" fight on later as early as 1922.
Quite a few in the military and government knew exactly what threat the Japanese were, and were already making plans.