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As I understand it there was information which might have led the US to realize that Japan was going to war with the US and quite possibly that Pearl was the target. I do not believe that there was a conspiracy just a bit of incompetence and a bit of bad luck. (Not unlike September 11)
Now what if folk had realized that a sudden attack was a real likelyhood and US forces were on full alert that Sunday morning.
I think it is quite possible that the Japanese attackers (who would have been towards the extreme of their range) could have been badly mauled.
Could the US have achieved a major victory?
Might the Japanese aircraft carriers have been detected and sunk?
Does this change everything?
(How big a risk is it that Hitler would have not declared war on the US?)