Florence, like all Italian cities, owed her independence to the duel of the Papacy and Empire. The transference of the imperial authority beyond the Alps had enabled the burghs of Lombardy and Tuscany to establish a form of self-government. This government was based upon the old municipal organisation of duumvirs and decemvirs. It was, in fact, nothing more or less than a survival from the ancient Roman system. The proof of this was, that while vindicating their rights as towns, the free cities never questioned the validity of the imperial title. Even after the peace of Constance in 1183, when Frederick Barbarossa acknowledged their autonomy, they received within their walls a supreme magistrate, with power of life and death and ultimate appeal in all decisive questions, whose title of Potestà indicated that he represented the imperial power--Potestas. It was not by the assertion of any right, so much as by the growth of custom, and by the weakness of the Emperors, that in course of time each city became a sovereign State. The theoretical supremacy of the Empire prevented any other authority from taking the first place in Italy. On the other hand, the practical inefficiency of the Emperors to play their part encouraged the establishment of numerous minor powers amenable to no controlling discipline. The free cities derived their strength from industry, and had nothing in common with the nobles of the surrounding country. Broadly speaking, the population of the towns included what remained in Italy of the old Roman people. This Roman stock was nowhere stronger than in Florence and Venice--Florence defended from barbarian incursions by her mountains and marshes, Venice by the isolation of her lagoons. The nobles, on the contrary, were mostly of foreign origin--Germans, Franks, and Lombards, who had established themselves as feudal lords in castles apart from the cities. The force which the burghs acquired as industrial communities was soon turned against these nobles. The larger cities, like Milan and Florence, began to make war upon the lords of castles, and to absorb into their own territory the small towns and villages around them. Thus in the social economy of the Italians there were two antagonistic elements ready to range themselves beneath any banners that should give the form of legitimate warfare to their mutual hostility. It was the policy of the Church in the twelfth century to support the cause of the cities, using them as a weapon against the Empire, and stimulating the growing ambition of the burghers. In this way Italy came to be divided into the two world-famous factions known as Guelf and Ghibelline. The struggle between Guelf and Ghibelline was the struggle of the Papacy for the depression of the Empire, the struggle of the great burghs face to face with feudalism, the struggle of the old Italie stock enclosed in cities with the foreign nobles established in fortresses. When the