The central theme of the Aeneid is the founding of Rome as the work of destiny, paid for with the suffering of those who carry it out: a man obeys an order greater than his will, and the poem shows, book by book, what that obedience costs.
The proem already says so. The poet sings of arms and of the man who, "a fugitive by fate," came from the shores of Troy to Italy: he does not choose the journey, he is pushed along by it. Jupiter promises the Romans an empire "without limit of space or of time," and Anchises sums up what Rome must do: rule the peoples, impose the custom of peace, spare the conquered, and beat down the proud.
Around this axis turn the lesser themes. Duty against desire, when Aeneas leaves Dido to sail for Italy. Destiny and resistance to it, in the figure of Juno, who knows she will be defeated and fights anyway. Exile and the search for a home, from Troy to Latium, until Aeneas says before an unknown shore: "Here is the home, this is the homeland." And war, which the poem tells in the last six books.
The poem does not celebrate without putting the cost in view. Dido, Pallas, Camilla, Euryalus, and Turnus die along the way to the founding. In the parade of future Romans, among the praises of Augustus, Anchises warns against civil war and mourns the young Marcellus, whom the fates will show to the earth without letting him stay. The same phrases serve as the justification of dominion and as the measure of its price, and the text does not separate them.