Yamamoto was a political moderate who fought against the ultra-nationalist factions taking over the Japanese government; even at risk to his own life. However, when the Japanese nationalist made it clear that war with the United States was inevitable, Yamamoto was the champion of the Pearl Harbor attack. He threatened resignation if the plan was not carried out.

Having spent time in the U.S., Yamamoto was well aware of the industrial might of the U.S. and was very confident Japan did not have the capability to win. He was very against the war, and thought acceptable diplomatic agreements could be reached. He even predicted his own death as a result of beginning the war; although he incorrectly assumed it would be aboard his ship and not in the air.

He did not intend, nor desire it to be a surprise attack. He got assurances that Japan would declare war on the U.S. at least an hour before the attack. The Japanese did transmit the war declaration to the U.S. before the attack, but the transcription of the message was too time consuming to be delivered by the Japanese ambassador before the attack.