What if the Japanese carrier fleet were spotted a day before reaching Pearl Harbor?
Short answerThe US fleet could mobilize and put to sea, creating a possible carrier or surface battle whose outcome would depend heavily on who found whom first.
NotesA full day's warning could get the US Pacific Fleet out of harbour and turn the attack into an open-ocean engagement. Mobilisation prevents the historical strike against ships concentrated at their berths, but it does not remove the danger. If American forces know the Japanese position without revealing their own, they can try to strike first. If the Japanese locate the fleet first, the offshore battle could still produce severe American losses.