In terms of their fatalities, the relative magnitude of the first World Trade Center bombing to the Oklahoma City attack was less than that of the Oklahoma City attack to 9/11.

Certainly, 9/11 wouldn't have had such a powerful effect on the public if the attack weren't of such great magnitude. But external threats tend to unite groups much more powerfully than internal ones, and the positive feedback from that makes hate spirals much easier. If we had been confronted with a threat which we couldn't externalize as a monolithic enemy, and were instead given a clear case of terrorism as the product of an ideological extreme within a society, the one in which we ourselves lived, the idea of declaring a state of war on terrorism would have looked far less credible.