69. The only hope for the Lombard towns was in _union_, which the emperor had explicitly forbidden. Soon after Milan's destruction measures were secretly taken to form the nucleus of what became later the great Lombard League. Cremona, Brescia, Mantua, and Bergamo joined together against the emperor. Encouraged by the pope and aided by the League, Milan was speedily rebuilt. Frederick, who had been engaged in conquering Rome with a view of placing an anti-pope on the throne of St. Peter, was glad, in 1167, to escape the combined dangers of Roman fever and the wrath of the towns and get back to Germany. The League was extended to include Verona, Piacenza, Parma, and eventually many other towns. It was even deemed best to construct an entirely new town, with a view of harboring forces to oppose the emperor on his return, and Alessandria remains a lasting testimonial to the energy and coöperative spirit of the League. The new town got its name from the League's ally, Pope Alexander III, one of the most conspicuous among the papal opponents of the German kings. [Sidenote: Frederick completely defeated by the League at Legnano, 1176.] After several years spent in regulating affairs in Germany, Frederick again appeared in Lombardy. He found the new "straw" town, as the imperialists contemptuously called it, too strong for him. The League got its forces together, and a great battle took place at Legnano in 1176,--a really decisive conflict, which was rare enough in the Middle Ages. Frederick had been unable to get the reënforcements he wished from across the Alps, and, under the energetic leadership of Milan, the League so completely and hopelessly defeated him that the question of the mastery in Lombardy was settled for some time. [Sidenote: Peace of Constance (1183) establishes independence of Lombard towns.] A great congress was thereupon assembled at Venice, and here, under the auspices of Pope Alexander III, a truce was concluded, which was made a perpetual peace at Constance in 1183. The towns received back practically all their regalia and, upon formally acknowledging the emperor's overlordship, were left by him to go their own way. Frederick was forced, moreover, humbly to recognize a pope that he had solemnly sworn should never be obeyed by him. The pope and the towns had made common cause and enjoyed a common victory. [Sidenote: Origin of the power of the Guelfs.]