The common thread of all the Cantos 6: Inferno, Purgatory, and Paradise is political. Dante speaks to three people from Italy who expound upon politics in Italy. In Inferno, Dante speaks to Ciacco who foretells the fate of Florence and its division and Dante’s exile. Dante compares Florence to Gomorrah pondering where he might find some just citizens that it be spared.
In Purgatory Dante speaks to Sordello who speaks a condemnation of Italy and Florence. Much like that found in the book of Lamentations. Bemoaning Jerusalem.
In Paradise, Dante speaks to Justinian who expounds upon the whole history of the Roman Empire and how it leads up to the birth of Christ and the just rule of Christ. Also defending that all authority is under the Providence and authority of God.
Though history is wrought with injustice and destruction, all history is moving toward something, an end point.
Cantos 6 are all preludes to Christ–the end of the world. All things before Christ were leading up to Christ the King of kings who is the Just Ruler of all. Who fulfills all that human history was seeking.
(some insights gleaned from Tony Esolen)