It's clear an attack on Pearl Harbor, had been a concern, for 20 yrs before it actully happened. George Patton wrote a paper, in the early 1930's, that "outlined almost exactly the plan used by the Japanese" (source, pattonhq.com/pattonbio.pdf). Several simulations had suggested the possiblity as well. Many were also concerned about the large concentration of ships and other high value military assets, all in one place. Some of which could have been, kept on the west coast, further away from the likely enemy. We know Adm Richardson was fired for voicing his concerns, about the lack of anti-aircraft weapons and pursuit aircraft (fighters), needed to protect the Pac Fleet. (source, ibiblio.org/pha/pha/misc/defense.html). It's also important to understand, Adm Nimitz, turned down this coveted command, only to accept it after the war started. After the attack, in the Knox Investigation, these same issues come up. (source, ibiblio.org/pha/pha/knox/knox_sec.html) "In view of the attack and the serious damage inflicted by it, the usefulness and availability of the Naval station must be restudied. Its air defenses must be strengthened immediately by the despatch of as many fighter planes and anti-aircraft guns as can be assigned to it. Special defenses against aerial torpedoes, such as balloon barrages and deep floats to be moored alongside important combatant units must be developed. Pending studies and the addition of satisfactory safeguards, no large concentration of Naval vessels can be permitted at Pearl Harbor." The other very important, info in the Knox Investigation, is the "Activities of Japanese fifth columnists' and the spying going on. "It cannot be too strongly emphasized that there was available to the enemy in Oahu probably the most efficient fifth column to be found anywhere in the American possessions, due to the presence of very large numbers of alien Japanese. The intelligence work done by this fifth column before the attack provided the Japanese navy with exact knowledge of all necessary details to plan the attack. This included exact charts showing customary position of ships when in Pearl Harbor." From this it's clear, the Pearl Harbor commanders were facing both an external and internal threat. A threat that was not know and could only be estimated. No wonder Nimitz turned down the job. From the Naval Court of Inquiry, many other problems with the fleet being a Pearl Harbor are expressed. Like the high land around PH, preventing visability of approaching hostile planes and helped the spying activities, that were going on before the PH attack. Along with how barrage ballons and smoke, as means of defence, could not be used. Due to navy airfields near-by and the prevailing winds. The Naval Court of Inquiry, goes on to give many other reasons, the disaster took place. Adm Stark not keep Adm Kimmel informed, about the diplomatic situation. How Gen Marshall and Adm Stark sent a joint memo to FDR, on 5 Nov, 1941, not to give the Japanese an ultimatum. Because the ultimatum would mean war and the US military was not ready for war, and the Japanese had an existing numerical superiority, in terms of their fleet. The Naval Court of Inquiry, may well be the most objective, investigation into the PH disaster. It also explains the reason, no long distance recon was being done at the time. How the Army was responsible for the AWS (Aircraft Warning Service) as part of the, JOINT COASTAL DEFENSE PLAN. This should have given warning, of approaching hostile aircraft, and in fact it did. However, there was a failure of execution, after the detection was made. The ships in the harbor had ammo, and were firing in about 5 mins, after the start of the attack. Had they been given more time to get ready, they may have been able to drive off, the 1st and 2nd waves of planes, like they did the 3rd. I also read some where that Japanese cruiser recon planes, were also detected, before 7AM, and also dismissed as our own planes, by more experienced officers, than Lt Tyler. Didn't seem like anyone really took a possible attack seriously, in either Pearl Harbor or Washington DC. The vulnerability of ships to air power, just seemed to be a lesson that had to be learned the hard way. Considering Force Z, lacking any air power, was sent anyway and quickly destroyed.