“If we had believed the intel we had in hand in 1941, there would have been no Pearl Harbor!”
There would still have been a Pearl Harbor. It may not have turned out as badly as it did, or possibly it may have turned out worse.
Strategically, the US knew that Japan was preparing to attack - somewhere. War warnings went out on Nov 27.
“Message Sent by Navy Department, 27 November 1941:
This dispatch is to be considered a war warning. Negotiations with Japan looking toward
stabilization of the conditions in the Pacific have ceased and an aggressive move is
expected with the next few days. The number and equipment of Japanese troops and the
organization of the naval task forces indicates an amphibious expedition against either the
Philippines, Thai, Kra Peninsula, or possibly Borneo. Execute an appropriate defensive
deployment preparatory to carrying out the tasks assigned in War Plan 46 [The Navy’s
war plan]. inform district and army authorities. A similar warning is being sent by the
War Department. “
The problems were that :
a. There was no specific warning in re Pearl Harbor, though the possibility of that had been well understood for years.
b. The local command, on many levels, was lackadaisical and fundamentally unprepared for war. There were few revetments at the airfields, interceptors were unprepared, planes were (foolishly) not dispersed, even with thorough knowledge of WWII experience, and even lacking effective radar there should have been effective standing patrols and an observer network, which there weren’t. Ships and bases did not have their AA armament ready and manned at dawn.
c. The Japanese attacked with overwhelming force, which the US forces in Oahu could not have effectively opposed, even if they had been prepared. It would have required US rearmament to have begun a year earlier than it did to even those odds.
d. Had the fleet sortied to avoid being trapped in port they might well have suffered worse casualties, as they would have been sunk at sea by a massive air attack that they had no way to oppose. Those 21-knot “standard” battleships of the line were just torpedo magnets.