Ishmael and Isaac are pictures of the city of man and the City of God. Yet God is sovereign over both of them, Ishmael by nature and in the city of man, Isaac by grace and the promise and in the City of God.
The city of man is fleeting and will be punished, "nevertheless, while history lasts, it has a finality of its own." Despite the fact that it seems to be always at war with itself, in fact it pursues peace and what it believes to be the highest good. This highest good is pride, which is the source of all contention and war in the world and the source of the lust for power and domination.
For all this, we say that civilization is a good thing: "It is wrong to deny that the aims of human civilization are good, for this is the highest end that mankind of itself can achieve.... The things of earth are not merely good; they are undoubtedly gifts from God."
Yet, even as peace is pursued misery is the result, since the peace in question is pursued through war, domination, pride, and all manner of sin.
Even the origins of the city of man have to do with sin, as Cain killed his righteous brother Abel. Why should we be surprised that the whole long history of man is so sordid and wicked? It was founded in blood and will continue in blood until it finally receives its deserved punishment. Until then, the city of man will hate the City of God simply for being what it is: "the root of the trouble [between Cain and Abel] was that diabolical envy which moves evil men to hate those who are good for no other reason than that they are good."
This is seen to be especially wicked when we realize that good is something that can be shared with no diminishment [compare this to the first part of Augustine's On Christian Doctrine, where he makes the same statement about Scripture interpretation]. Cain then gains nothing by slaughtering Abel, though he could have had the goodness at no cost to Abel and great benefit to himself. "What is more, goodness is not merely a possession that no one can maintain who is unwilling to share it, but it is one that increases the more its possessor loves to share it." Abel, Augustine assumes, would have delighted to bless his brother with the truth and been the better for doing so.
So we have the wicked fighting the wicked, and the wicked fighting the good; and because of continuing indwelling sin, we likewise have the good sometimes fighting the good [I like to think that Augustine has the Donatists in mind here]; how much more should we long for the City of God, where true peace will be established!