Virgil wrote the Aeneid during a period when Roman territory had just experienced significant expansion, first under Julius Caesar, and then, more recently, under Caesar Augustus (a.k.a. Octavian), the first Roman Emperor. Just as importantly, Augustus had also consolidated political authority in himself, putting an end to the years of brutal civil war that followed the death of Julius Caesar. The spirit of these heady days is reflected in Jupiter and Anchises's predictions that Roman power will expand to the limits of the earth. (This contradicts Jupiter's idea that the Romans will have "empire without end" in space, though it might still be unlimited in time.) And yet, many years before the birth of Peter Parker, a.k.a. Spiderman, the Aeneid argues that "with great power comes great responsibility." This can be seen in Anchises's instructions to Aeneas that he must not only "battle down the proud" but also "spare the conquered." Among the positive aspects of Roman power, as depicted in the Aeneid, is the lasting peace (eventually known as the Pax Romana) it brought to the various countries under its dominion.
Virgil thinks empire is justified only if it brings peace.
Virgil's epic shows empathy for the powerless.