A major asset of al-Qaida was that it possessed a highly dispersed force that enabled it to group and regroup. It had demonstrated the ability to operate globally.
This was a force that could not be rapidly defeated. Nor could it be negotiated with or even located for negotiations.
The United States could not accept the status quo. Al-Qaida had demonstrated its capabilities (on 9/11), and there was no reason it would not strike again. Lacking political solutions, Washington’s only option was a military strike – a broad and diffuse campaign designed to fragment al-Qaida.
Invading Afghanistan and Iraq was the only practical option if the goal was to cripple a very capable enemy. (snip) It was an unconventional counteroffensive, and this is what its critics dislike, but they offer no clear alternative. After 9/11, the threat was simply too great. The strategy was worldwide disruption. It was not pretty, but it worked. There were no other large-scale attacks on the U.S. homeland.
Friedman makes a good argument. I am willing to let history judge this invasion, having mixed feelings about it myself.