We even have examples in our own times of the defeat of great powers against all odds and clearly by the decree of God. It seems that a particular Gothic king (whom you can read about here, if you can stand some dry historical prose) intended to sacrifice the leadership of the Roman Empire to the pagan gods. He was defeated nearly miraculously by Stilicho, a barbarian (or at least half-barbarian) who had risen through the ranks to command the remnants of the once-mighty Roman legions. Augustine points out that in that case, even the pagans celebrated the defeat of Rome's enemy, when in reality they should have wished for a victory to 'prove' the dominance of the pagan gods.
The greatest power and authority imaginable in Augustine's time--the office of Roman Emperor--is insufficient to convey true happiness. That comes alone through the worship of the true God and through living a godly life.
But we say that they are happy if they rule justly; if they are not lifted up amid the praises of those who pay them sublime honors, and the obsequiousness of those who salute them with an excessive humility, but remember that they are men; if they make their power the handmaid of His majesty by using it for the greatest possible extension of His worship; if they fear, love, worship God; if more than their own they love that kingdom in which they are not afraid to have partners; if they are slow to punish, ready to pardon; if they apply that punishment as necessary to government and defence of the republic, and not in order to gratify their own enmity; if they grant pardon, not that iniquity may go unpunished, but with the hope that the transgressor may amend his ways; if they compensate with the lenity of mercy and the liberality of benevolence for whatever severity they may be compelled to decree; if their luxury is as much restrained as it might have been unrestrained; if they prefer to govern depraved desires rather than any nation whatever; and if they do all these things, not through ardent desire of empty glory, but through love of eternal felicity, not neglecting to offer to the true God, who is their God, for their sins, the sacrifices of humility, contrition, and prayer.Even success as an Emperor will provide neither satisfaction nor fulfillment, that comes only in repentance and faith in the Gospel of Jesus Christ.
As with Augustine's discussion of evil in Book I, we see that he believes God is careful to neither bless too much nor bring too much suffering lest we begin to worship Him for those reasons alone. Constantine was given power and allowed to rule as a Christian monarch for so long to provide evidence that it is possible to be a Christian and a political ruler at the same time--one need not worship demons in order to achieve worldly glory. Yet, lest we replace "demons" with "God" and worship only to achieve worldly glory, we have the examples of Jovian, who by all accounts was a faithful Christian and yet reigned less then a year and died in a very unglamorous way, and Gratian, who ruled longer than Jovian but died by the sword.