While the Ultranationalists in Tokyo dreamed of samurai spirit defeating battleships, Yamamoto was looking at spreadsheets of Pittsburgh steel production.
Isoroku Yamamoto wasn't just a visitor to the United States; he was a student of its power. As a naval attaché and later a student at Harvard, he didn't just study naval tactics. He studied Detroit. He studied the oil fields of Texas.
He hitchhiked across the country. He played poker with Americans (and usually won). He understood something his colleagues in Tokyo did not: Americans were not weak "merchant people" who would fold at the first sign of aggression. They were sleeping giants of industrial manufacturing.
// YAMAMOTO'S WARNING (1940)
"Anyone who has seen the auto factories in Detroit and the oil fields in Texas knows that Japan lacks the national power for a naval race with America."
Yamamoto opposed the Tripartite Pact with Germany. He opposed the invasion of China. He knew that adding the US to Japan's enemies list was suicide. His opposition was so vocal that the Army plotted to assassinate him.
To save his life, the Navy Minister moved him from Tokyo (where he was vulnerable) to sea, making him Commander-in-Chief of the Combined Fleet. Ideally, keeping him safe on a battleship.
"To fight the United States is like fighting the whole world. But it has been decided. So I will fight the best I can. And I will die on board the Nagato."
— Letter to a friend, 1941
If war was inevitable (as the Army insisted), Yamamoto concluded there was only one path to survival: A Knockout Blow.
The US Pacific Fleet outnumbered Japan. A traditional defensive war would slowly bleed Japan dry.
Destroy the US battleships at anchor in Hawaii. Break American morale instantly. Force a negotiation within 6 months.
It required sailing 6 aircraft carriers 4,000 miles across the North Pacific undetected. Never in history had a fleet of that size moved that far in secret.