What Yamamoto meant by the WH comment was actually super humble:
He meant that if Japan fought the USA at all, the notion that the USA would fight until some brokered political settlement was fanciful.
He was saying that Japan would have to win so utterly in order for the USA to stop fighting that Japan would have had to take CONTINENTAL US territory —his meaning was the exact opposite of what people made it out to be.
He studied at Harvard, and had driven his American convertible all over the USA, admiring the oil industry here.
THAT is why he was fixated on the question of oil & energy —as a navy man he knew the stuff was the lifeblood of combat ops.
In fact he was quite an AmericanoPhile.
A couple things:
1. The Japanese Navy had a deep schizm; On the one hand there were the older admirals who loved battleships and saw aircraft carriers are uppity contraptions not to be trusted.
On the other hand were folks like Yamamoto —the CARRIER was the new center of navy power.
PH was an air operation, and Yamamoto was in ultimate charge. However Yamamoto was not actually ON SCENE —the person on-scene was a Battleship guy (Nagumo) who still didn’t fully accept the crazy idea of airplanes landing on ships —after the 2nd wave got back safe and sound, he ordered the fleet to withdraw.
In fact all the aircraft carrier people violently urged a 3rd (and even more) waves —they were amazed with their level of success and saw the inertia still in their hands. They wanted to exploit it even further —how about the dry docks? How about the oil facilities? These were left untouched.
If they didn’t have the crusty battleship admirals along, what would have happened?
My view is in fact the US got off MUCH easier than could have been the case.
2. Almost ALL Japanese Navy admirals expected that after the PH attack the fleet would engage in ship-to-ship combat with the US navy immediately off of Hawaii and they FULLY EXPECTED to lose 50% OF THEIR ATTACKING FLEET..!!!
Their level of air success was amazing, but what amazed them even more was that there was NO navy engagement —they got away scott-free.