No they weren't. They didn't have nearly enough alert aircraft to do anything. There were only 21 aircraft for the entire US that morning that were
ready to launch, and capable of doing something.
Why bother sinking billions a year into "defense budget" when they can't even be counted on to protect us from a few jetliners? Hell, those could
have been some military jets dropping bombs, think of the damage they could have done then...
If they were military aircraft coming from the outside, they would have been ready for it. They would have seen it coming from a long way out. They
weren't though. They were civilian airliners, hijacked, already in the US. The military was always designed to stop incoming threats, not threats
that started inside the US.