Vigilante groups usually arise in weak states with deteriorating security situations in which locals feel compelled to take their security into their own hands. The state will often try to co-opt these militias, who are afforded a level of legitimacy in their communities that the military is not, at least initially, and thus have a distinct advantage in counterinsurgency operations.

The U.S. government spends billions to promote democracy. It could get more for its money through better internal coordination, and by relying more on NGOs and its own expertise when implementing pro…