I don't see any overriding objection to trying Khalid Sheikh Mohammed, or others alleged to be involved in the planning of the 9/11 attacks, in civilian courts. Neither does Matt Yglesias. But this argument of his in favour of a law-enforcement rather than a war-waging approach to the prosecution of terrorists is, in my view, a bad one:
In political terms, the right likes the war idea because it involves taking terrorism more "seriously." But in doing so, you partake of way too much of the terrorists' narrative about themselves. It's their conceit, after all, that blowing up a bomb in a train station and killing a few hundred random commuters is an act of war. And war is a socially sanctioned form of activity, generally held to be a legally and morally acceptable framework in which to kill people. What we want to say, however, is that this sporadic commuter-killing isn't a kind of war, it's an act of murder. To be sure, not an ordinary murder - a mass murder - but nonetheless murder.
The opposition Matt sets up here between war and crime - between 'a socially sanctioned form of activity, generally held to be a legally and morally acceptable framework in which to kill people', on the one hand, and murder or mass murder, on the other - is too sharp. War may (sometimes) be legally and morally acceptable, but that doesn't mean there is no criminality within war. There is - as defined by the laws of war. One is not therefore bound to choose between treating individuals as participating in a war and treating them as criminals, if that is what they are. Under the assumption of universal jurisdiction, international humanitarian law allows for war criminals and those responsible for crimes against humanity to be prosecuted in the civilian courts of any country. And terrorism is murder even when it is 'a kind of war'.