Could they have known? Those 19 crazed, homicidal humans, drunk on religion and righteousness, could they have know just how much they'd change the world? I doubt it, though I suspect they hoped as much.
I'm not naive, nor am I unaware of the world outside North America. People all over the world lived with fear and uncertainty long before that that one day in September where just how violent a world we live in was driven home . Hell, there are even plenty of people, the destitute and desperate poor of North America, who knew this fact long before that day. The ability to turn groups of humans into the "other", into less than you are, is all that's needed to commit atrocities. Once you've done that, the rest is just planning and logistics.
I wish to live in a world where the only requirements to be ingroup is to have human DNA and act as if all people are of equal worth. I don't even care that everyone actually believes that we're all equal, just that they act as if we are. And isn't it a condemnation of the society in which I live that I feel the need to define what "all people" means?
To this end, I beg you, the next time you find yourself using the word "they", stop. Stop and think about the group you're talking about. Think about why you've grouped them together. Take a moment to remember that no matter how different you think they are, they're all still human. They had a mother and a father, they eat and sleep and desire. They are most likely more similar to you than they are different. And yes, I did this when I thought about the 911 hijackers. Remembering that they are human too does not excuse what they did. Remembering their humanity reminds me that I too can make people into the "other". It reminds me that I have to better than that.
That's the lesson of 911 for me. I will remember.