Really?
Japanese fleet out on recon is engaged and attacked by the US Fleet, where was the "sneak attack"? It seems the US was the ones who ordered the sneak attack.
How would we have known they were on their way here, Sunshine? That would have tipped our hand that we cracked their codes, no?
So all ships and personnel are removed from Pearl Harbor, the attack goes in and does no significant damage, and the ships are attacked and sunk after that. Simple enough.
The point is that we lost most of our battleships in that attack. Now, you know and I know that battleships were dinosaurs in World War II and aircraft carriers and submarines were the important ships, but FDR didn't know that -- most naval opinion hadn't caught up yet, and he was far from a naval-tactics visionary. If he had known what was coming, he'd have saved the battleships, and he'd have sunk the enemy fleet. He didn't. Therefore, he didn't know.