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 was confined to intermittent sorties of Holy Roman Emperors. Renaissance politics developed from this background. Since the 13th century, as armies became primarily composed of mercenaries, prosperous city-states could field considerable forces, despite their low populations. In the course of the 15th century, the most powerful city-states annexed their smaller neighbours. Florence took Pisa in 1406, Venice captured Padua and Verona, while the Duchy of Milan annexed a number of nearby areas including Pavia and Parma.
