Augustine recaps the first books, and then explains why he is going into so much historical narrative: it is because the two cities (that of man and that of God) exist side-by-side in the world, and the city of man has mostly overshadowed the history of the City of God. Augustine "wanted to bring it into the light during the period which begins with God's more outspoken promises and ends with their fulfillment in Christ's birth of the Virgin."
In this book, however, he is going to go back into the city of man and give us a rough outline of its development.
Despite all the variety among men, there is only one city of man. "The simple truth is that the bond of a common nature makes all human beings one." Of course, this is not to say that there is no diversity:
Nevertheless, each individual in this community is driven by his passions to pursue his private purposes. Unfortunately, the objects of these purposes are such that no one person (let alone, the world community) can ever be wholly satisfied. The reason for this is that nothing but Absolute Being can satisfy human nature. The result is that the city of man remains in a chronic condition of civil war.We're always trying to conquer others, and while God directs in his "mighty providence" who will win and who will lose in these conquests, they never truly satisfy us. We see this in the examples of the two mightiest empires the earth has seen to date--Assyria in the East and Rome in the West. Because Romans are so unfamiliar with Assyria, Augustine is going to share some highlights from its history to show how it is the exemplar of the city of man--all other kingdoms are in a sense merely appendages to these two great powerhouses.