In the Aeneid, fate (or destiny) is an all-powerful force—what fate decrees will happen, must happen. It is Aeneas's fate to found a city in Italy, and so that he will do. Characters can, and do, have the free will to resist fate. But ultimately, such resistance is futile. Juno can delay Aeneas reaching Latium for a while, but not forever. Dido can get Aeneas to stay in Carthage for a while, but not forever. Turnus can fight Aeneas off temporarily, but not forever. And while, for the gods, resistance to fate seldom seems to have consequences, for mortals such as Dido and Turnus, efforts to resist fate end disastrously, suggesting that resistance to fate is seen in a negative light. Though the predestined fates may seem to kill the suspense of the storyline, there's a different kind of drama at work in the Aeneid—whether and how the characters accept their fates, and in the particulars of their journeys to fulfilling their fates.
The theme of fate also helps to link the story of Aeneas with the real-life time of Augustus Caesar, who ruled the Roman Empire when the Aeneid was written. Aeneas's destiny is to begin the civilization that will become Rome, and to begin the line of kings that will result in Augustus. Therefore, the poem endows Augustus's government with invulnerable, divinely sanctioned power: Augustus was fated to rule, in a destiny that stretches all the way back to his great ancestor! Anchises makes this point clear in the Underworld, when he shows Rome's future leaders to Aeneas. Fate justifies not only the poem's plot, but also Augustus's government.