Scratch the surface of most of the world’s armed conflicts and you won’t see an ideological struggle or deep-seated ethnic or religious hatred that has erupted in a war of all against all. Rather, what you’ll find is a collection of thugs for hire, some in uniforms and some not, who are taking advantage of the chaos of war to prey on the weak. Sometimes this involves stealing oil or diamonds, or sexual brutality. But it’s pretty rare that it involves some larger design. True, the hooligans who take part in these orgies of destruction will often claim loyalty to...