Brink Lindsey has a great series started on terrorism. Part 1 covers the history of conflict between settlers and warriors (who I agree are closely tied to terrorists), and Part 2 talks about how the complexity of modern society … and the high levels of interpersonal trust that make that complexity possible … are a key part of our vulnerability to terrorism.
Glenn Reynolds referred me, and leverages off this to talk more about the potential strength of packs (as opposed to crowds) in defending us against this, and I agree with him (and want to extend the discussion to talk, like a good liberal about how the government could encourage and harness this).
Lindsey is sounding damn worried about these things:

Here is the gist of it: We find ourselves, once more, in that paradoxical vulnerability that our forebears suffered for more than 20 centuries. The old menace, long vanquished, has returned in new guise. We are threatened again by an enemy whose weaknesses in peace become strengths in war. Our civilization is exposed to ruin by the very sources of its greatness. After a long respite, the barbarians are at the gate again.
(from part 1)
Terrorism, of course, is nothing new: Its modern history dates back at least to 19th-century Russia. But the march of economic development and technological progress has, perversely, led to a qualitative increase in terrorism’s virulence. The power to inflict physical damage has grown by orders of magnitude, while the escalating intricacy of the division of labor means a similar, exponential increase in the economic and social costs associated with any particular act of physical destruction. As a result, the leading edges of civilization are now prone to outside attack for the first time in half a millennium. If we do not now take the full measure of this threat, and bend our considerable energies towards countering and neutralizing it, we are likely to pay a grievous price for our complacency.
(from part 2)

…and from my point of view, he’s absolutely right to be.
We have some key problems:
– the destruction of intermediate economic and social structures in both rich and poor societies, which lead in the short run to alienation and poverty in poor societies as traditional support structures are eroded, and alienation and anomie in richer societies as the spiritual and psychological support structures are eroded;
– the rapid and free movement of goods and people that has accompanied globalization increases the ability of destructive actors to get access to and gather resources for attacks;
– the wide diffusion of the ability to use, if not create, destructive technology – ranging from small arms to WMD;
– the increasing dependence of our economy and cultures on complex, easily damaged infrastructures.
Each of these suggests both that at this moment in history there are more threats, and that we are increasingly vulnerable to those threats.
Lenin suggested a long time ago that capitalists would sell the communists the ropes that would be used to hang them. Modern Western societies are inventing and manufacturing the technologies that will be used … hell, have been used … to attack us.
Our objective, as I note below, needs to be both to reduce the vulnerability to threat, and to reduce the level of threat.