I still wonder what effect terrorism would have on our country if it happened on a wide scale. While nothing minor, September 11th only impacted one city. If there was a large scale attack encompassing dozens of cities on a daily basis I think the impact would be much more dramatic.
The fact that terrorism finally hit on American soil has influenced American life. We used to believe that it couldn't happen here, as we watched it happen everywhere else. This is part of the reason we've been in two wars for the last decade.
It poses the same problems the American guerrilla warfare caused the British during the American Revolution. It gives an outmatched opponent the ability to engage and possibly defeat their enemy.
This forum isn't the place to make a laundry list of terrorist attacks, but overall, they were executed by people who were outmatched so they resorted to unconventional tactics. The challenge moving forward is how to engage and prevent these unconventional attacks and protect the populace.
The major challenge that is presented is that of how to combat international terrorism. The terror from the early part of this period went away on its own, really, without us doing much to stop it. So we never really learned how to deal with it. We don't know today how to best deal with the current version of international terrorism.