Everyone in America who was alive on this day 20 years ago knows where they were on that day. Those who were born since then can know about it only second hand. But much of what they grew up with was shaped by that day.
What have we learned since then? Surely that to defend our country and to punish those who would do or attempt to do such unspeakable things is crucial. Keeping that the focus, however, is key, and not allowing that mission to creep into massive efforts at nations building - staying within the confines of the necessary and possible and not attempting the desirable but impossible.
Make America as free as it can be, a beacon for all of the oppressed of the world. Freedom and prosperity are more powerful and enduring forces for good than are bombs and invasions.