In the final part of book on Augustine discusses another legendary Roman, Scipio Nasica, the Pagan High Priest and Senator that opposed the destruction of Carthago. Augustine's take on this complicates my interpretation in the previous post. Scipio's point was that the romans would be corrupted if they did not have a strong enemy.
For Scipio feared security as the enemy of weak spirits. He saw that fear was necessary to the citizens: to act, as it were, as a suitable tutor during their pupillage. Nor was he mistaken in his judgement; for the outcome proved how truly he had spoken. For when Carthago was destroyed and the great terror of the Roman commonwealth thereby repulsed and extinguished, the prosperous condition of things immideately gave rise to great evils.The lack of an outer enemy lead the romans into civil wars and more importantly "lust for mastery" and the avarice and luxury.
Now, I don't know about you, but to me this sounds more like the arguing of a fascist than of a Christian. And Augustine keeps this up, he nowhere (at least not in book I) criticises this way of reasoning. In my dissertation I show how Isaac of Nineveh interprates fear in a similar way but draws the opposite conclusion from it. Empire needs fear to function - it is the way the citizens are controlled. But for Christians this means that we have to denounce empire and let Christ free us from all fear. In fact, Augustine here does not at all problematize the connection of Christianity to empire he is merely concerned with pointing out that the Romans were not corrupted by Christianity but by their own power befor the rise of Christianity. It is as if he is arguing against Gibbon! (also, it is a weak argument - Rome did not really lack enemies outside its borders during the time when Roman virtues were weakned...)
Isaac's view is far better I think, and still, of course, perfectly valid today.
Update: In IV, 3 Augustine has a more negative view of fear: "The wealthy man, however, is troubled by fears; he pines with grief, he burns with greed." It seems to bea feature of Augustine's rhetorics that he feigns agreement on his supposed opponents in all issues but the one discussed at the moment.