Matt Yglesias asks MR to address whether we would prefer, all other things equal, terrorists organized into a single group, or organized into competing groups. The answer to this question will not be a priori, but here are a few relevant considerations:
1. If terrorists perform their acts for fundraising purposes, or for criminal status, you would probably rather face a monopoly opponent. They are more likely to rest on their laurels.
2. If you think that terrorists are deterrable, at least in principle, you would prefer an easily identifiable monopoly opponent.
3. If you think that terrorists are likely to engage in internecine warfare with each other, you would prefer the more competitive set-up. (I’ll predict that if anyone kills, or has killed, bin Laden, it is one of his own people.) This is especially true when the terrorists are far away from you; they can fight without major spillover effects on your citizenry.
4. Perhaps the production of terrorist attacks involves significant economies of scale. In that case you would prefer the smaller competing groups. Nuclear weapons probably involve such economies, but suicide bombings can be organized on quite a small scale.
My guess: In Iraq you would prefer a smaller number of groups, since there is some chance of striking a deal with them. And there we are more worried about the suicide bombers than a loose nuclear device, so economies of scale do not overturn this conclusion. We are less likely to ever “trade” with al Qaeda and its offshoots, so in that case I would prefer splintering. Furthermore al Qaeda has a greater long-run nuclear potential, so it is more important to deny them potential economies of scale. I suspect we do not much mind if western Pakistan becomes a scene for terrorist infighting, whereas such conflicts could scuttle reconstruction in Iraq.