As best we know, Hirohito was timid, cautious and even dubious about going to war with the US.

Japan destroyed records after it surrendered, and MacArthur as the post-war administrator gave the entire imperial family immunity from war crimes prosecution so we have quite limited evidence of Hirohito's state of mind going into the war. This we do know.

On September 5, 1941, Hirohito received an outline for Japanese plans for war or peace. Hirohito was outraged that the war option was presented before the peace option. Hirohito demanded that the chief of staff of the army, Sugiyama, and chief of staff of the navy, Nagano, appear before him that day and explain themselves.

At that meeting Sugiyama assured Hirohito that the war would be won in a few months time. Growing angry, Hirohito reminded Sugiyama that he had said the same thing about how long it would take to win a war with China, and that was 4 years ago, and no end to that war was in sight. Hirohito scolded him, "You say the interior of China is huge; isn't the Pacific Ocean even bigger than China? Didn't I caution you each time about those matters? Sugiyama, are you lying to me?"

Admiral Nagano told a trusted colleague, "I have never seen the Emperor reprimand us in such a manner, his face turning red and raising his voice."

The next day, at the formal meeting to adopt the plan, everyone in attendance advised the Emperor to adopt the plan which favored war as a solution rather than peace. Hirohito acceded. He did however take the unusual step of making a statement at the end of the meeting in which he stressed the need for diplomatic solutions, and he read a poem about international peace.

At some times after that meeting he reportedly tried to encourage another dove about the war, saying that the war would not be so bad, things will be okay. We don't know whether Hirohito changed his mind about the war after September 6 and before December 7. We just don't have any evidence.

Japanese culture then (and now) strove for group assent and collective decision making. The military, which ran the country, wanted war as did the Japanese people who had been subjected to years of propaganda. And Hirohito saw himself as a constitutional monarch who does not rule alone but only with the rest of the government and with the will of the people.

Hirohito's War: The Pacific War 1941-1945 by Francis Pike.