The theme of Fate is hugely important in the Aeneid. Heck, it seems like every five minutes we're being reminded that the Trojans are going to found a new city in Italy. When we see the souls of future Roman heroes in the underworld, waiting to be born, or the exciting images of Roman history on Aeneas's shield, these strongly suggest that the Trojans are going to be successful (because otherwise, you know, there'd be some kind of weird time-paradox going on). You might think that this takes away from the poem's suspense, but that's kind of missing the point. You see, the ancients had a pretty nuanced view of Fate. As the goddess Juno never gets tired of reminding us in the Aeneid, destiny may determine that the Trojans will found a city in Italy, but it doesn't stipulate how they end up doing it. Juno uses that as her angle to give the Trojans an incredible amount of trouble. The flip-side of this is that, even though the ancients believed in Fate, this didn't mean that they disbelieved in Free Will. Thus, when Aeneas tells Dido, "I sail for Italy not of my own free will," he doesn't mean that the Fates are forcing him to go there. What he means is, he has an obligation to go there, which he is choosing to live up to. For more on this issue, see the theme of Duty.
In the Aeneid, fate plays less of a role than characters' beliefs about what their fate will be.
The Aeneid portrays free will as potentially dangerous.