I was struck, during the media wind-up for September 11th this year, by repeated references to the Twin Towers attack as "America's Darkest Day".
Now, I'm pretty sure that if you asked someone what the "darkest days" of Germany were, they'd start talking about Nazis - similarly if you tried the same trick re: Cambodia you'd get a mention of the Khmer Rouge. You wouldn't in either case, expect the response to be an account of the harms done to those countries by forces outside them.
By this logic, it would seem more appropriate candidates for "America's Darkest Day" might be the 6th and 9th of August, 1945. On these two days, the US dropped atomic bombs on Hiroshima and Nagasaki. The immediate death toll for the Twin Towers attacks was about 4.2% of the instantaneous loss of life which happened in Hiroshima - between 70 and 80 thousand people killed immediately, and the same amount again injured.
You can make the point that this was an act of war, but I'd argue that the September 11th attackers certainly viewed themselves as soldiers. I'd also point out that, while the Twin Towers were indisputably civilian targets, any attack where 30% of a city's population is instantly wiped out is automatically going to count as "terrorising the civilian population".
If you think that raising 70 year old history is unfair, I'd concede you probably have a point. That said, the US as recently as the 1980s and 90s was willing to bankroll people who were definitely terrorists, so long as they terrorised people the government of the day disapproved of, or conducted their terrorism in the service of a useful regime.
These are all points you've heard before - usually raised by people who want to claim that America somehow "deserved" the September 11th attacks. As I said in my post on Osama bin Laden, I'm uncomfortable with the idea that anyone ever deserves death. I also think that it's unhelpful to get too caught up in the idea of global terrorist networks as somehow being a consequence of America's foreign policies and cultural attitudes.
A more useful point to make (and one that might be more conducive to reflective moments in the corridors of power if it became a meme) is that death and terror are commonplace for many people in the world today. This is not a good thing.
Until September 11 2001, that was something that happened "somewhere else" for most Americans - even when it happened next door in Mexico and Central America. This was actually a fairly unique position at the time, even amongst First World countries. The UK had Ireland to worry about, and European countries had a variety of homegrown radicals. I don't think there's any particular virtue in being periodically carbombed, but I suspect that it's much harder to maintain a solipsistic view of your own country and its politics under such circumstances.
The Twin Towers attacks were not exceptional in their savagery, their death toll or their targeting of innocents. They were unique because they represented an attack on the previously-invulnerable USA. September 11th 2001 was not America's Darkest Day, it was The Day America Joined the Rest of the World.