Sorry, but I have to clear something up…
Isoroku Yamamoto knew that Japan would lose a war with the United States. He said so on several occasions before the war, and lost his cabinet position because of it. He studied at Harvard, and was a military attache to Washington D.C., and was very familiar with our industrial potential.
Yamamoto is often described as a "dove." He opposed the 1931 invasion of Manchuria, the war with China in 1937, and the Tripartite Pact with Germany and Italy. There was once even a plot on the part of Japanese militants to assassinate him.
He opposed war with the United States but once his country was committed (by Tojo and his flunkies) he felt that he had to fight to the best of his ability. And he fought very well.
That's why WE assassinated him.
He WAS the architect of the Pearl Harbor attack. (And we can debate the morality of that somewhere else…) But, the impression of him as a warmonger was wholly a product of allied propaganda. Tojo was the warmonger.
But, yes, Tojo did underestimate the American "public." One of his arguments for war was that Americans lacked any warrior spirit, and would decline to make war on Japan beyond a token effort. (Which is ridiculous, considering the warlike history of the United States to that point.)