Yamamoto appreciated the irony: having risked his life to prevent war with the United States, he was now its architect. “What a strange position I find myself in,” he wrote a friend, “having been assigned the mission diametrically opposed to my own personal opinion, with no choice but to push full speed in pursuance of that mission. Alas, is that fate?”
And yet even in the final weeks of peace, Yamamoto continued to urge that the wiser course was not to fight the United States at all. “We must not start a war with so little a chance of success,” he told Admiral Nagano. He recommended abrogating the Tripartite Pact and pulling Japanese troops out of China. Finally, he hoped that the emperor would intervene with a “sacred decision” against war. But the emperor remained silent.
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"In the first six to twelve months of a war with the United States and Great Britain I will run wild and win victory upon victory. But then, if the war continues after that, I have no expectation of success."
This prediction turned out to be very spot on and prophetic. The Battle of the Coral Sea happened in May 1942 and was a similar trade of blows. Fast forward to June of 1942, 6 months after Pearl Harbor, and we see the beginning of the end.
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"Should hostilities once break out between Japan and the United States, it is not enough that we take Guam and the Philippines, nor even Hawaii and San Francisco. To make victory certain, we would have to march into Washington and dictate the terms of peace in the White House. I wonder if our politicians, among whom armchair arguments about war are being glibly bandied about in the name of state politics, have confidence as to the final outcome and are prepared to make the necessary sacrifices."
Yamamoto was put between a rock and a hard place. He didn't advocate for the attack on Pearl Harbor because it he supported it, he advocated it because it was the best "least worst" decision Japan could make in his eyes and potentially cripple America militarily to buy time, hopefully getting out of the war. Unfortunately for Yamamoto he was correct tactically about Pearl Harbor, but incorrect strategically. He didn't want war with America, but his position meant that if it was going to come to it (and it would) that he was responsible for finding the best method to serve his country. Which, at the time knowing what was available, was the attack on Pearl Harbor. Unless he could've gotten the military to pull out of the war, which wasn't going to happen.
Also interesting, there was a naval wargame done at the Japanese Navy War College in 1927 where Japanese carriers would lead a strike on Pearl Harbor. Which... considering the time and technology is pretty wild to even consider it. Yamamoto was not involved in that, but spoke of it later and knew about it in the following years.
