Not a complete surprise, it was half an answer to Roosevelt's problem.

The US knew an attack was coming, but expected it in the Phillipines or Singapore. Most important people stayed on duty in Washington that weekend.

So, it was easy to declare war on Japan. But what about Germany? It was not involved, and there was no excuse for a fight against her.

Well, Germany and Japan had a mutual defense treaty providing that an attack against one was an attack against both. Except the the US didn't attack Japan. That was presented to Hitler as a real good reason to not go to war with the US. Hitler considered it for a while and then decided what the hell.

He declared war on the US on December 11, 1941, sealing the doom of the Third Reich.