Why was the USA so unprepared for the attack on Pearl Harbor in 1941?

Extracts from an American report on the attack on Pearl Harbor in 1941

Catalogue ref: WO 208/2081

The underlying cause of this error of judgement was General Short’s confidence that Japan would not then attack Pearl Harbor. In fairness to him it must be borne in mind that this belief was shared in by almost everyone concerned including his superior officers in the War Department in Washington. He was undoubtedly influenced in such a belief by the then prevailing psychology which completely underestimated the Japanese military capabilities and particularly the advance which they had made in the use of aircraft. General Short also knew that the Naval command at Hawaii, which he regarded as being better informed than he because of their facilities and the widespread nature of their operations, was confident that an air attack on Pearl Harbor was most unlikely.

To sum up the situation tersely, General Short was warned by Washington that there was immediate danger both of an attack from without by Japan and of an attack from within by sabotage. This warning required him to be alert against both forms of danger. He chose to concentrate himself so entirely upon a defence against sabotage as to leave himself more completely exposed to an attack from without than if there bad been no alert at all. He so concentrated his planes as not only to make them an easy target for an attack from without but to require several hours to get any substantial number of them into the air for defense.

To such an error of judgement it is no excuse that he relied upon assurances from another service, even though he thought that that service was better informed than he was as to the disposition of the Japanese fleet. He was the responsible defender of the outpost of Hawaii. He bad no right entirely to subordinate his duty to be prepared against what he knew to be the most dangerous form of attack on that outpost to the opinion of another service.