Again, we see 'historical' proof (at least, accepted as history by the Romans) that the pagan gods do not even provide the peace that their worshipers seek when pursuing them. We have to remember the Christian doctrine of Providence, that "peace is a great benefit; but it is a benefit of the true God, which, like the sun, the rain, and other supports of life, is frequently conferred on the ungrateful and wicked."
In other words, the cessation of war is neither a sign of Divine favor nor evidence that the worship society is engaged in is the correct worship. It is merely (well, 'merely') the undeserved favor of God being temporarly poured out on a rebellious world.
But how do we respond to the argument that without its wars of expansion, Rome would never have been so great as it was (even if it has declined in modern times)? Augustine replies with the virtue of moderation: "why must a kingdom be distracted in order to be great? In this little world of man's body, is it not better to have a moderate stature, and health with it, than to attain the huge dimensions of a giant by unnatural torments, and when you attain it to find no rest, but to be pained the more in proportion to the size of your members?"
And this without even really discussing just how Rome got so big in the first place. I love Augustine's non-condemnation of the means: "But obviously the Romans have a plausible defense for undertaking and carrying on such disastrous wars--to wit, that the pressure of their enemies forced them to resist, so that they were compelled to fight, not by any greed of human applause, but by the necessity of protecting life and liberty."
In other words, the Romans never really wanted their huge Empire, they just kept having to defend themselves and, because they were so awesome, at the end of every war their state was that much bigger.
Whether that is true or not, the fact remains that the entire process of national expansion was nothing more than an open display of sin. The pursuit of empire, the luxury and opulence that corrupted the state once it was achieved, and the constant refusal to be content with what they had been given are pictures of how sin affects human life. We are never content with what we have, once we get what we desire it just makes us worse, and then we are ready to move on to new desires never once having stopped to reflect on our rebellion against our Creator, self-justifying the whole way.
Through all the processes of world history, the wars and conquests (whether Roman or Greek seems not to matter here, Augustine switches seamlessly between the two), we see at the very least that the pagan gods are impotent. Even studying the basics of history should be enough to convince the Romans of that!
If you're interested in some of the stories Augustine draws on in this section, they can be found in Virgil's Aeneid (the Fitzgerald translation is my favorite, but Fagles is quite good too); Sallust's Catiline's War (really a must-read for students of Ancient History); and Livy's History of Rome (probably longer than it's really worth, but at least sometimes interesting).