One of the earliest manifestations of municipal vitality was the war of city against city, which began to blaze with fury in the first half of the twelfth century, and endured so long as free towns lasted to perpetuate the conflict.  No sooner had the burghs established themselves beneath the presidency of their Consuls than they turned the arms they had acquired in the war of independence, against their neighbors.  The phenomenon was not confined to any single district.  It revealed a new necessity in the very constitution of the commonwealths.  Penned up within the narrow limits of their petty dependencies, throbbing with fresh life, overflowing with a populace inured to warfare, demanding channels for their energies in commerce, competing with each other on the paths of industry, they clashed in deadliest duels for breathing space and means of wealth.  The occasions that provoked one Commune to declare war upon its rival were trivial.  The animosity was internecine and persistent.  Life or death hung in the balance.  It was a conflict for ascendency that brought the sternest passions into play, and decided the survival of the fittest among hundreds of competing cities.  The deeply rooted jealousies of Roman and feudal centers, the recent partisanship of Papal and Imperial principles, imbittered this strife.  But what lay beneath all superficial causes of dissension was the economic struggle of communities, for whom the soil of Italy already had begun to seem too narrow.  So superabundant were the forces of her population, so vast were the energies emancipated by her attainment of municipal freedom, that this mighty mother of peoples could not afford equal sustenance to all her children.  New-born, they had to strangle one another as they hung upon the breast that gave them nourishment.  It was impossible for the Emperor to overlook the apparent anarchy of his fairest province.  Therefore, when Frederick Barbarossa was elected in 1152, his first thought was to reduce the Garden of the Empire to order.  Soon after his election he descended into Lombardy and formed two leagues among the cities of the North, the one headed by Pavia, the center of the abrogated kingdom, the other by Milan, who inherited the majesty of Rome and contained within her loins the future of Italian freedom.  It is not necessary to follow in detail the conflict of the Lombard burghs with Frederick, so enthusiastically described by their historian, Sismondi, It is enough for our present purpose to remember that in the course of that contention both leagues made common cause against the Emperor, drew the Pope Alexander III. into their quarrel, and at last in 1183, after the victory of Legnano had convinced Frederick of his weakness, extorted by the Peace of Constance privileges whereby their autonomy was amply guaranteed and recognized.  The advantages won by Milan who sustained the brunt of the imperial onslaughts, and by the splendor of her martyrdom surmounted the petty jealousies of her municipal rivals, were