Upvote:2
It was, of course, impossible for him to know what was in peoplesβ heads. He was making an assumption and providing leadership. He knew good and well that Japanβs international aggression - as well as Germanyβs - 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.