The problem here is that given patience and money, there is nothing that cant be done. Sorry, but a few clever individuals can pretty much do what they want if they are determined and smart. Gonna frisk everyone down? Check every trunk, van, suitcase before it enters a major city? Detectors? I can get around that. They can too.
So what to do? Nothing practical. Depopulate cities, telecommute, live underground. That is a start. Will anyone do this? Nope. As far as attacking a foreign country, who do you go after? Lift prints from a detonated bomb? Some say the truth will come out, but US citizens will cry out for blood before then. Anyones. Doesnt have to be who did it, but who we think OUGHT to have done it. In any case, suppose I were a domestic terrorist (no I am not) and decided to leave some clues pointing to an Arab state? We nuke the Arabs. Oops.
This is like living in the midwest. You know a tornado is going to wipe out your house one day. Could be in 1 day or 50 years, so life goes on. This is how I think Americans are responding psychologically