[Torplexed]

Interesting scenario. Even if the US had been able to react to the Japanese attack hours before it struck it might have still been a disaster of sorts. The US battleship fleet was hit at anchor in shallow water which meant all but two of them were salvaged. If they had been at sea they might not have been sighted, but if they had, they would have been sunk in deep water with greater loss of life. I always thought the wisest first move for the Japanese would have been to hit the Oahu airfields with everything they had, waited for the US fleet to sortie, than hit the fleet at sea with only minimal air cover left to oppose them. Given the average slow speed (21 knots) of the US battleships there is no realistic way they could have caught the faster Japanese carrier fleet in a surface action given the 200 nautical miles between them. Their best bet would have been to elude them by steaming south. Given the superior range of Japanese planes Nagumo's carriers could have stayed way well out of the range of an effective US land-based air strike.

Another thing to bear in mind is that the Japanese had a cloud of submarines posted around Oahu. They might have gotten in a few potshots at a departing US battlefleet, although I doubt if they would have scored many hits.

There were two US carriers in the area, Lexington and Enterprise, They were Admiral Nagumo's nagging worry. But stacked against six Japanese carriers the odds wouldn't have been good. Unlike Midway the Japanese knew there were US carriers in the neighborhood.

Probably the best "what-if" would have been if the Japanese planes arrived to find the harbor emptied of ships and the skies full of waiting fighters and AA defenses active. Would have given the US a chance to knock off a little of the cream of Japanese naval aviation. At that point Nagumo probably would have started sending out search planes to find the US fleet and then it gets speculative.