Brad DeLong asks, 'Doesn't Anybody Read Max Weber Anymore?' by which he means many things, all interesting but not, perhaps, all equally correct.
That the crew in charge thinks history is bunk, and Weber some damforeigner is all too likely. That tangled lines of command for the military are a bad thing, we can all agree. Whether this is quite as true for civilians in all cases, I'm not as sure; sometimes having criss-crossing lines can be an efficient way to move information around an organization.
But I'm least sure that I am prepared to say that either history or present experience teaches us to adopt the Roman, or British, proconsular model. In an era of modern communications, it's not necessarily wrong to have lines of authority run to HQ, nor is it necessarily wrong not to have the military report to the local viceroy. And it would be especially wrong, I think, to decide that the proconsul must be a military officer in order to unify the commands. Weber also taught us about bureaucratic virtues and there are more to them than clear lines of command and obeying orders, and while the Army has quite a few of these virtues, some of the ones that a civilian reconstruction project ought to care about are likely to go out the window in a theater of operations.
The root problem with the CPA was not, is not, that it lacks the ability to order troops around. The problem with the CPA is its (in)competence, the lack of planning before it started operations, and the very small number of officials who speak the local language or know the local culture.
If we are going to draw lessons from the British Empire — a very very mixed model if you were to ask me, or ask any number of colonized peoples, then the example I would choose to emulate is that of the district magistrate, oxford trained, fluent in three of the local languages.