Psalm 46: 9 (45:10 Vulg.), the Psalm that inspired “A Mighty Fortress Is Our God,” says: “He maketh wars to cease unto the end of the earth; he breaketh the bow, and cutteth the spear in sunder; he burneth the chariot in the fire.” In his commentary on the Psalms, the Explanatio Psalmorum, St. Ambrose interprets this verse as a prophecy of the Roman Empire and the Pax Augusta–that is, the Romans imposed peace upon the world, thus facilitating the missionary journeys of the Apostles. But, for Ambrose, that peace is not only a practical boon. For he sees a parallel between the order of earth and the order of heaven. Thus those same Apostles “taught all men living under one earthly empire to confess with faithful speech the empire of the one almighty God.”
It looks like there may be one English translation out there that includes St. Ambrose’s commentary on this Psalm. I’m not certain, and I haven’t seen it. But here is what he says on v. 9 (10).
And truly, before the Roman Empire spread its borders, not only were the kings of individual cities fighting with each other, but even the Romans themselves were often being exhausted civil wars. Marius fought against Cinna; Roman blood was spilled on each side. Sulla rose up and roused civil wars against the victorious Marius in turn. Lepidus and Sertorius acted as rebels against the Roman Empire. Caesar hunted Pompey and stirred up the rage of the Gauls against Roman arms. When he had conquered the elder Pompey, he waged war against the younger one in Spain. Why should I speak of the triumvirs, who from enemies became friends, and from friends went on to enemy tumults again, and of the sea dyed with the blood of Romans in the Battle of Actium? From this event it came about that the Romans, wearied by civil wars, bestowed the Roman Empire on Augustus, and in this way internal battles were pacified. Moreover, it had the following profitable result as well: the Apostles could be sent straight on through the whole world, when the Lord Jesus said: “Go and teach all nations.” To them, it is true, even realms cut off by barbarian upheavals lay open (as, for example, India to Thomas and Persia to Matthew); but nevertheless, in order that they might travel to more areas of the earth, at the time of the beginning of the church he spread the power of the Roman Empire through the whole world and reduced to order the minds of those who were at variance with each other and the divisions of land, having given them the gift of peace. They taught all men living under one earthly empire to confess with faithful speech the empire of the one almighty God.