At short notice? Yes. There's no such thing as 100% protection. A peace-time domestic-originating threat to the Capitol, from the air, was not considered likely. The cost of maintaining protection for such a highly unlikely threat would be enormous. The military does not have endless funds.
Until 0903 no one on the ground had any idea the US was facing a coordinated terrorist attack. At that time all strikes had been against New York. It was not until 0921 that the military or FAA had any notion that Washington DC might be a target.
NORAD were not notified of the hijacking of UA175 until 0903:
The United States military's power is in force projection - their ability to bring military power to any corner of the globe within a matter of days. Most of their operational military assets are focused on this role - carrier task groups, Marine expeditionary forces, the 82nd Airborne, special operations, and so forth.
NORAD has two roles - a relatively minor peace time role and a much more extensive role in the event of a full scale attack. The 9/11 attacks fell between these two - a peace time, domestic originated, small scale attack. NORAD were not designed to deal with that sort of threat, and were not capable of dealing with it.
As for "vital assets", I would say the US military's most vital assets are their carrier battle groups, all of which are protected 24/7 by a fighter CAP. None of the US military's major commands are based at the Pentagon.
I don't think you understand the impossibility of what you're expecting to have happened. NORAD had two pairs of fighters to cover the entire north east sea board. Their role in hijackings was strictly limited to escort and observation duties.
I don't know where your 48 minutes comes from, but 0921 was the first time NORAD had any awareness that Washington DC might be a target. That's not 48 minutes warning, that's 16 minutes warning. NORAD duties required aircraft to be airbourne 15 minutes after a scramble was issued. Find me a fighter that can fly from Langley to Washington DC, locate and intercept a low-level airliner travelling at 400MPH, ascertain that airliner has hostile intent, engage the airliner and shoot it down, all in 60 seconds, and you may have a point.
In my view you're letting you Hollywood-informed expectations cloud your understanding of how the real world operates.
It was Congress that clipped NORAD's wings.
NORAD had five fighters in the air less than an hour after they knew something was happening. By two hours, there were CAPs over New York City and Washington DC with as many as a dozen fighters airbourne.
By mid afternoon NORAD had 300 fighters in the air supported by AWACS and tankers, had implemented a modified version of the wartime SCATANA Plan, and had CAPs over every major US city.
I don't think you appreciate just how phenomenal an achievement that was.
Nonsense. This Cessna penetrated Soviet airspace from
outside. The aircraft on 9/11 didn't even have to do that. Secondly, the Russians protected their airspace
much more harshly than the US does. Simply compare the number of commercial airliners the Russians have shot down for entering their airspace unannounced with the number the US has shot down.
The vast majority of the fighters based around the US are for training purposes or for defending the USA in the event of a full scale attack. During peace, when there's no threat of an air attack, those defenses are not on standby, because frankly maintaining them on standby is flat out impossible. Instead a skeleton crew of fourteen fighters was maintained at seven bases around the perimeter of the contiguous USA, tasked primarily with intercepting drug smugglers trying to sneak through the ADIZ down near Florida, and occasionally popping up to investigate commercial airliners coming from overseas who had suffered radio or transponder failures.
That's all they were there to do. That has
always been NORAD's mission, for over fifty years.
You don't seem to appreciate the logistical difficulty of maintaining the sort of defenses you expect. I suggest you do some reading up on Operation Noble Eagle, and in particular the enormous strain this put on the part time pilots and ground crew of the Air National Guard.
It's simply not feasible. No country in the world can maintain what you expect during peace time.
-Gumboot