Commentary Par XXXI 31-40

In this lengthy simile the poet compares barbarians, probably coming, in times of peace, from Northern Europe to Rome, seeing the imperial buildings of the city before Constantine gave those buildings to the papacy just after his conversion in 312, to himself, moving in the opposite direction, 'south' to 'north,' from Florence to the New Jerusalem above the heavens.  The magnificent church of St. John Lateran was destroyed by fire in 1308.  Making things worse, Henry VII, denied a coronation in St. Peter's by Pope Clement, was crowned in the ruins of that church in 1312, nearly exactly one thousand years later, and died the next year (see C.Par.XVII.82-84).

For a discussion of the various notions of what exactly Dante meant by the reference, see Costa (Cost.1996.1), pp. 65-66.