I've been thinking of civilian control of the military. Obviously, it's a good thing. But I suppose one underlying original motivation was, in a general way, the sense that the military couldn't be trusted. Without civilian control, the generals would get too aggressive and the country would be stuck with wars it didn't want.
Seems to me now that, oddly, the opposite has happened. It is the civilians who are more warlike than the military. I don't think the military would have made the decision to invade Iraq (obviously none of the various reasons given by the administration--weapons of mass destruction, connections between Saddam and Al Quaeda, etc.--have any military significance). But the civilians did, they interfered with the military's war plans (remember Rummy deciding that we didn't need so many troops there?), and now the military finds itself where it shouldn't be and, I think, in the kind of war it doesn't want to fight.