So you actually believe that the Americans would ignore the warnings and let their personnel be slaughtered as a reason to enter the war?
Interesting, or are you being obtuse?

They might of course have not believed the implications or the significance of the information. Is that not a possibility?
Some Japanese destroyers against the might of the Pacific Fleet.

Ever since Japan's imperial ambitions in Manchuria had resulted in a blockade of fuel and metals the Americans knew some form of retaliation was likely. In the lead up to the attack there was plenty of intelligence. Indeed, the problem was that there was too much. US codebreakers had cracked the Japanese diplomatic traffic (but, crucially, NOT the naval codes) and an alert warning had been issued to all US Pacific outposts - including Pearl Harbour. Unfortunately, both the Navy and advisors in the government considered an attack on Pearl Harbour unlikely (given the enormous logistical problems the Japanese just barely managed to overcome). Worse still, the local commander thought so too, and tragically chose not to follow the explicit orders stating that every effort must be taken to defend the base from attack. He was court-martialed (along with several other senior naval commanders) for negligence soon after.

So, yes - the Americans knew an attack was likely. But no, they did not pull the trigger on their own people.

9/11 anyone?

Correct!

Human error is the common denominator, nothing more.
Hindsight of course helps the analysis.

Well the 'error' being in a) having intelligence that such an attack was being contemplated but doing nothing; b) cutting back on intelligence work related to al-Qaeda.

A flying school being told by a middle eastern man that he wasn't bothered about being able to 'land' was also a clue

Not 'arf!

IIRC, that was reported to the authorities.

The conspiracy stuff is utterly ridiculous – not least because the mix of incompetence and stupidity alluded to above say it's not required.

Indeed.

The Yanks changed the protocol in June of that year, prior to 9/11, and in doing so made it so that rogue airplanes needed central authorisation before being shot down. In the official report they made a point of that saying that the previous protocol (to intercept the plane) would have led to attempt at bringing the planes down. I believe that both Rumsfield and Cheney (certainly the latter was anyway) were involved in changing that protocol.