These things being so, we do not attribute the power of giving kingdoms and empires to any save to the true God, who gives happiness in the kingdom of heaven to the pious alone, but gives kingly power on earth both to the pious and the impious, as it may please Him, whose good pleasure is always just. For though we have said something about the principles which guide His administration, in so far as it has seemed good to Him to explain it, nevertheless it is too much for us, and far surpasses our strength, to discuss the hidden things of men’s hearts, and by a clear examination to determine the merits of various kingdoms. He, therefore, who is the one true God, who never leaves the human race without just judgment and help, gave a kingdom to the Romans when He would, and as great as He would, as He did also to the Assyrians, and even the Persians...Hopefully this is both self-explanatory and evident to all believers who acknowledge the sovereignty of God in the affairs of man: kingdoms are given and taken away by the Providence of God alone. They are neither rewards for human virtue nor signs of Divine favor. And they are certainly not signs of salvation--God by grace alone gives that particular gift, while he gives political rule to the saved and unsaved alike. We can know that this giving of political power is just and according to the wisdom and plan of God, but saying anything more than that "is too much for us, and far surpasses our strength." When we try we're attempted to peek into the secret things of God, and that is beyond our ability.
Augustine follows this statement with a list of examples, various kingdoms who worship different 'gods' and various emperors who were sometimes good and sometimes evil, and even the Christian Emperor Constantine and, after his time, the pagan Emperor Julian. (Whether Constantine was a Christian is now a hotly debated topic: no one doubts Julian's pagan devotion.) The point is, we should not equate political power with either God's favor or proof of salvation. No doubt every American needs to hear this: we are not guaranteed political power, and a revival of our faith is no promise at all that we will receive such. God has a plan in store for America that may involve intense persecution of Christians, or the rise of faithful believers to positions of power, or anything in between. At no point, however, are we told anything about what will happen other than that God is in control of it.
Even war is under God's sovereign authority.
Thus also the durations of wars are determined by Him as He may see meet, according to His righteous will, and pleasure, and mercy, to afflict or to console the human race, so that they are sometimes of longer, sometimes of shorter duration.The very worst of possible circumstances in the ancient world--a long war followed by conquest and subjection to an enemy--are controlled by the will and power of God. We should not read a national loss of a war as a mark against Christianity, nor a national victory as a mark for it. Augustine reminds his readers that there were long and shorts wars and victories and losses for the Romans long before the Christians were on the scene.
These things I mention, because many, ignorant of past things, and some also dissimulating what they know, if in Christian times they see any war protracted a little longer than they expected, straightway make a fierce and insolent attack on our religion, exclaiming that, but for it, the deities would have been supplicated still, according to ancient rites; and then, by that bravery of the Romans, which, with the help of Mars and Bellona, speedily brought to an end such great wars, this war also would be speedily terminated. Let them, therefore, who have read history recollect what long-continued wars, having various issues and entailing woeful slaughter, were waged by the ancient Romans, in accordance with the general truth that the earth, like the tempestuous deep, is subject to agitations from tempests—tempests of such evils, in various degrees,—and let them sometimes confess what they do not like to own, and not, by madly speaking against God, destroy themselves and deceive the ignorant.It is not the fault of Christians or Christianity if a nation--be it Rome or America--is winning or losing a war. Turning to other gods or belief systems may indeed have a practical effect on the war (say one embraces a pacifist religion, that may change the outcome for obvious reasons), but in the end God remains sovereign and the course of human events remains under His authority.