I don't have my copies of the books anymore, but didn't Debt of Honour end with an airliner being flown into Washington D.C.? Seems pretty real to me.
MAJOR HUGE GIANT SPOILER FOR DEBT OF HONOUR
Yes, it might, except that the pilot took off from Canada, not Dulles (or whatever it is called now) or Boston or New Jersey. The flight plan was not pre-approved, like the 9/11 planes. It was hijacked by the pilot in Vancouver, taken to Montreal, and then flown down the east coast on an illegal flight path. It was on the illegal flight path for several hours before slamming into the Capitol Building, during a joint session of Congress, killing almost all of Congress, the President, the cabinet and the Supreme Court.
The 9/11 planes were in the air for only minutes before they struck their targets, and they started off as approved flights, before altering for only a number of minutes before hitting their targets. Also, killing almost all of the US government in one fatal blow, with perfect timing? With a no-fly zone over DC proper? Patrolled by the US Air Force? It goes far beyond the attacks in 9/11, which is why I found the ending a stretch. Not to mention, I read the book in the early 1990's, when the idea seemed pretty inconceivable. Maybe that carried over to my comment above, as it is certainly more conceivable today, but the circumstances Clancy sets out are pretty far fetched. The ending allows Clancy to set up government as he sees fit in the next book
(fewer politicians, more real Americans in congress). Something of a deux et machina, in my opinion.
This is why the earlier books seemed better to me. They had plausible circumstances, whereas as the series progresses, the circumstances get less and less plausible, and fit Clancy's more right-wing, big military, idealism.