I think it really was a surprise.
We suspected they might pull something like this but not exactly when.
That being said, I believe it was imperative for the US to enter the war. I can't imagine if Germany, Japan, & Italy had won, what the world would have looked like afterwards.
Toland contends President FDR knew.
Prange contends FDR did not.
I thought it was telling that Prange basically argued: FDR did not know, but even if he did, it was a good thing because we needed to get into the war.
I think FDR knew we were going to be attacked, but never believed the Japanese could hit us so hard and do so much damage. Before Pearl Harbor, and even for a short while later, Churchill and FDR very much underestimated the Japanese.
The British navy paid a terrible price with the sinking of the Prince of Wales and and the Repulse. Churchill sent them in the South Pacific without any air cover, The sinking occurred on 10 December, 1941, three days after Pearl Harbor.