To this day I am not convinced the U.S. didn't know it was coming. I believe there were high ranking people in the government that wanted to enter WWII but the public was against it and they were not prepared, having only about 100,000 troops ready to fight. Pearl Harbor solved all of that. You now had your "Look they started it" moment to justify getting in, and the patriotism that arose compelled hundreds of thousands to sign up for the military.
But hey, maybe its true that they thought the blips on the radar coming towards Pearl Harbor that morning was actually a flock of birds