A genocide-prevention division within the U.S. Army would circumvent this problem. Since its only mission would be to stop genocides, deploying the force would never require us to ask more of soldiers who already have their hands full with other conflicts. Moreover, those volunteering for the new force would know exactly what they were getting into and enlist specifically because they embraced the mission. These soldiers could be recruited from the ranks of idealistic college and high school students across the nation who have done so much to keep Darfur in the public eye.
I think O'Hanlon is a little over-optimistic about how the division would play out in real life, but I think there is some merit to the idea. Matt Yglesias, by contrast, is not as enthusiastic:
Color me skeptical. Different kinds of soldiers get different kinds of training, but they're all at least semi-fungible. If we had a spare genocide-prevention division lying around, it would be getting sent to Iraq as part of the "surge" not to Africa. The President would simply argue that escalation of the Iraq War is a genocide-prevention mission because of the sectarian violence. Then on the flipside, I'm not sure there's a discrete military task called "genocide prevention." You might, in an effort to halt a genocide, bomb some buildings or troop formations somewhere. Alternatively, as part of a war to overthrow the Taliban you might wind up policing the streets of Kabul and taking responsibility for the safety of the city's residents. So you want some military forces who specialize in bombing, and others who specialize in policing, but you don't have some troops who specialize in genocide prevention and others who specialize in attacking hostile governments.
I disagree that "genocide prevention" (or perhaps, intervention), is not necessarily a discrete task from other military activities. I seem to recall, in the wake of the Iraq fiasco, several calls for the US to develop dedicated "peacekeeping forces", under the theory that the training required to "shock and awe" a defending army is not the same as is required for counter-insurgency, peacekeeping, and reconstruction. I believe that stopping a genocide--which is less about crushing an opposing army than it is about securing civilians--probably requires a different skill set and mentality (building trust with the locals, negotiation to end the conflict, strengthening long term institutions) than the regular army has. This isn't to say that a genocide division would never need to be supplimented by regular forces (such as air support), but I do think it would serve a unique role. And of course, just having a division with anti-genocide work as its specific role makes it more likely we will actually engage in such interventions.
As to Yglesias' worry that such a division would never be able to be completely separated from regular army work, he's probably right that this is a risk, but I'm not sure I see it as controlling. If we're going to send 20,000 troops as a "surge" in Iraq anyway, it doesn't really matter where in the army we get them from. Compared to the increase (however marginal) in likelihood that we will effectively intervene to stop genocide, I think that it's worth the chance.