The Aeneid is about Rome, obviously.
I think Virgil’s epic breaks the idea of Rome into two primary categories: power and love. First, let’s think about love. Virgil uses romantic love to show us the way our individual desires can be fickle. Aeneas loves his wife Creusa, but then he loves Dido, and then he loves Lavinia. Romantic love is not trustworthy, Virgil suggests. True “love” for Virgil seems to be, instead, a desire for the good of the greater whole. In this way, love is political, not personal. Aeneas’s proper desire is for Italy/Rome. Lavinia represents this proper direction of desire. She’s hardly a character; she’s more of an ideal.
This view of love leads us now to a discussion of power. Virgil is on Ceasar’s side when he hints to us that true “love” should be directed toward the greater good of Rome. Yet Virgil also seems to be suspicious of the way expressions of power can go over the top (and in this way he might be undercutting Caesar). I take the end of Book 12 as a reminder that Roman power should be weilded in moderation. And while I do think Virgil sees personal sacrifice as necessary for the greater good, it is, he shows us, still painful. Romans, in this view, should remember that their greatness is built on top of much suffering, so expressions of Roman power should be restrained by this sad recognition. I think this understanding explains the tragic thread we find woven through the poem.