And yet, we still have the idea at the core of it all that FDR believed the Congress would not declare war without a devestating defeat and major loss of life.
That alone requires a Reynolds Wrap beannie.
No one knew that Japanese torpedoes would function in Pearl Harbor, which was believed too shallow to defend against the threat.
The extent of the success of the Japanese attack need not have been predicted for the “Roosevelt knew” hypothesis to be entertained, only the likelihood that Roosevelt may have wished for the Japanese clearly to strike first.
Do you not know the history of that time?
You don't remember that in 1940, FDR ran for reelection to his third term, and what did he solemnly promise the American people?
Come on! You should be able to repeat this by heart:
"I have said this before, but I shall say it again and again and again; your boys are not going to be sent into any foreign wars."
- FDR, Speech in Boston, October 30, 1940.
Here's another along that same vein:
"And I can cite to you many, many other examples of rumors that have been started in the same rumor factory, all of them untrue, but every one of them tending to make people believe that this country is going to war.
Your President says this country is not going to war."
FDR - Remarks at Buffalo, New York November 2, 1940
There are a number of examples of German U-boats sinking US ships in the Atlantic, which got Roosevelt all excited, but Congress was unmoved.
Most important point: there is no evidence -- zero, zip, nada -- that FDR or any other American, expected the massive damage that Japanese were able to inflict on Dec 7.
So, I think they expected some kind of attack, but not what they got.