The answer to this question can be found in Book 12. As Aeneas and his forces prepare for battle, he makes a treaty with Latinus and his people, clearly stating that they will fight together and that they will share the same fate after the battle, either in success or victory. Note the holy vow that Aeneas makes before the gods where he agrees this:
But if victory agrees that our contest is mine (as I think
more likely, and may the gods by their will prove it so),
I will not command the Italians to submit to Trojans nor do I
seek a kingdom for myself: let both nations, undefeated,
put in place an eternal treaty. I will permit your gods
and their rites: Latinus my father-in-law will keep his weapons,
my father-in-law will keep his accustomed power: the Trojans
will build walls for me, and Lavinia will give her name to a city.
The honour that Aeneas ascribes to Latinus and the Italians clearly acknowledges their position as an important ally for Aeneas and his men. Consider how he chooses to praise Lavinia by giving her name to a city. Aeneas thus finds that in the final battle he has to face he is not alone, and that he can depend on Latinus and his people, the Italians, as he goes into battle. This is the important ally that Aeneas makes, and it is the support of this ally that greatly helps him towards victory.