The reason it took 9/11 was that event made it clear that the world is a dangerous place, there are people who want to do us harm, we have to protect ourselves.
Prior to 9/11, Americans lived in a fantasy world where Americans believed that no one would hurt us because all we wanted was to be their friend. American felt that if Americans played nice with other countries that those other countries would be nice to Americans. Americans also assumed that the vast, beneficent government would protect them from all harm and they did not need to protect themselves.
The events of 9/11 disabused rational Americans of these silly, childish, dangerous, and ultimately fatal notions.
Now, Americans realize it's a dangerous world. They realize that we are responsible for protecting ourselves and that an armed populace is the first line of defense against our enemies.
Thanks for the article, btw. It's encouraging, but we still have a long way to go.