"Both of his victory and the papal mantle"

(Canto 2, line 27)

In book 6 of the Aeneid, Aeneas meets his father Anchises' soul in the underworld, who reveals to him how he will defeat Turnus of the Rutuli and others. Dante's point is that Aeneas' rise to prominence in Italy enabled the founding of the Roman empire and eventually, the emergence of papal power (the "mantle").