I don't think this is quite true.
Surely an important factor here is that the Twin Towers burned for over an hour on live rolling news broadcasts before collapsing and one of the two towers was hit by a second plane live on TV. I think the collapses themselves and the pictures of people throwing themselves off the building created some of the more horrifying scenes that will stay in the memories of everyone who witnessed the attacks.
It is too abstract to say that it was because the Twin Towers were full of civilians and the Pentagon was full of military men and women. If the Twin Towers had been attacked in just the same way and had been a military base then I think people would still have ended up with the same emotional impact. Remember that the plane which flew into the Pentagon was also full of civilians and the bombs that ripped apart the US embassy in Nairobi killed 200 civilians yet the thing that probably makes it different is the live broadcasts of the attacks. The fact that everyone witnessed them.
For this reason also, then. An attack on Pearl Harbor may not have had the impact that the attack on the Twin Towers had because it is unlikely that we would have the same images from such an attack.
I respect your disagreement, and I also see your point about the TV images. No doubt that they were a large component of the psychological wounding. I fully agree that it's a large factor.
That said, I in turn don't agree with your dismissal of civillian and military distinction. Remember: We
have incidents of prior attacks to study, and I do see more anger and horror being displayed at civillian targets over military ones. When you look at the USS Cole attack, the Kenyan embassy bombing, etc., none of those impacted as much as even the WTC bombing in the 90s (years before September 11, 2001). The attacks the public has always viewed as horrific were ones like the Bali bombings, the Lockerbie 747 bomb, back in the 80's the Berlin discotheque bombing, and so on.
Take this pair of executions by terrorists as a test case: Leon Klinghoffer and Robert Stenthem. Both were horrific events. Both actually occurred within months of each other. Both were of course by Islamic terrorists (Palestinian ones in the former, Hezbollah in the latter). And both were on US tourists that happened to be on the cruise (former) or flight (latter) that was targeted. Yet, which victim is recalled more as symbolic of the sort of horror that terrorism represented? As a single anecdote speaking towards this: I've discovered that
a musical tragedy has been written about one of these victims, and it wasn't the one who was a Navy Seabee.
Another casual measure (admittedly non rigorous) is the simple number of news hits relative to each other. I find more on Klinghoffer. And I'm tempted to go to the local university library and make a more rigorous measure of this, see how many feature articles are written on one compared to the other. That would be a mild measure of popular interest in one case over the other.
I think the trend would hold true for the reporting and feature column apearances for other events, and the dividing line would be between civillian and military targets. I further think it might be possible to gauge outrage by analyzing the verbiage in respective articles, although that starts to wander into subjective measurements, so maybe it wouldn't be a good idea. Regardless, this could be the beginning of a testable hypothesis. But getting back on track: My own perceptions has been that more outrage has been spent over the civillian aspects than the military ones. Looking at 9/11 but past the New York events: I see more general outrage at the Flight 93 crash than I do the Flight 77 crash. Part of that of course is the narrative: Passengers attempted to retake UA93, whereas AA77's passengers were not recorded doing anything similar. But I cannot separate out the fact that Flight 93's popular, public narrative centers around the civillian passengers far more than Flight 77's does, and I attribute much of that to the fact that UA93 didn't hit anything but a field, therefore the spotlight is on the passengers reactions. Whereas there were actually
more civillians killed in the Flight 77 attack (
"53 passengers and 6 crew members aboard American Airlines Flight 77; 33 passengers and 7 crew members aboard United Airlines Flight 93"... and that doesn't include civillians on the ground in the Pentagon (70 killed)), but the narrative revolves around the fact that it hit
the Pentagon. And on top of all that, there's a very evocative, emotive movie on UA93, but none on AA77. UA93 seems to resonate, seems to "hurt" more, so to speak than Flight 77 does. And this in spite of the fact that you can argue the Pentagon event had a far bigger civillian impact than the Shanksville one. Yet it's Flight 93 that gets the movie and also ensconced in society's memory.
I don't think the distinction can be dismissed easily. I don't think it's necessarily
right that people don't view attacks on military buildings and targets as less horrific - those are still citizens, still living, breathing people there, they're simply in uniform - but I
do think it's one of the fundamental distinctions being made by the general public. And I think a study of past terrorist events bears this out.