The main historical significance of 9/11 is that it served as a pretext for a stepwise escalation in US fascist narratives & policies, as part of the right's decades-long project to remake the country as a blood-and-soil homeland.
It was one more piece of the permission structure.
Basically post-WWII US history has been the long process of the US fascist movement, which had been decisively suppressed & rejected as part of the global fight for democracy, reconstituting itself & slowly but surely bending the country back to its will.
They used 9/11 as an excuse to accelerate, sure, but they use every major historical event that way. Carter's hostage crisis, Afghanistan withdrawal, the election of a black president, take your pick -- they're always pushing, pushing, pushing in the same direction, so any event is an excuse.