Northern Italy and upper Central Italy were divided into a number of warring city-states, the most powerful being Milan, Florence, Pisa, Siena, Genoa, Ferrara, Mantua, Verona and Venice. High Medieval Northern Italy was further divided by the long-running battle for supremacy between the forces of the Papacy and of the Holy Roman Empire: each city aligned itself with one faction or the other, yet was divided internally between the two warring parties, Guelfs and Ghibellines. Warfare between the states was common, invasion from outside Italy confined to intermittent sorties of Holy Roman Emperors.
