December 7th, 1941, was the date that was to live in infamy, as mentioned by F.D. Roosevelt in his Pearl Harbor speech. Overall, the speech was quite straightforward in its goal: assurance to the public that USA would take all measures to defend itself and make sure that such treachery would never occur again. However, he mentioned how the Americans had already formed their opinions. This creates curiosity as to how was the president so sure of the opinions of the people of USA just a day after the attack? Moreover, what was the reaction in public after the attacks and did the people support taking up arms?
It was, of course, impossible for him to know what was in peoples’ heads. He was making an assumption and providing leadership. He knew good and well that Japan’s international aggression - as well as Germany’s - would ultimately lead to conflict. He was trying to edge the country toward war, in spite of a strong isolationist movement in the US.
Pearl Harbor gave Roosevelt the casus beli he needed to bring America into the war. I think he could also rely on the reaction that Americans would have to a sneak attack that took place during negotiations with the Japanese Empire.