Sure the US may be past it's glory days. Hell even the Rand Corporation (who write a bunch of stuff for govt leaders and other high ups) says it's been trending downhill since some point in the early 2000s. They didn't mention 9/11 but it seems like a good historical milestone.
Essentially the paper says the last 200 years have been an anomaly and we're slowly sliding back to historical norms. They call it the neomedieval era and it's not just the US.