league of cities in northern Italy that, in the 12th and 13th centuries, resisted attempts by the Holy Roman emperors to reduce the liberties and jurisdiction of the communes of Lombardy. Originally formed for a period of 20 years on Dec. 1, 1167, the Lombard League initially consisted of 16 cities, later expanded to 20, including Milan, Venice, Mantua, Padua, Brescia, and Lodi. It was backed from its beginning by Pope Alexander III, who saw in it a welcome ally against his enemy the Holy Roman emperor Frederick I Barbarossa. Frederick suffered several military setbacks at the hands of the league, notably the Battle of Legnano (1176), and, after a six-year truce (1177–83), agreed to the Peace of Constance, by which he retained the fealty of the Lombard cities but granted them communal liberties and jurisdiction.

The Lombard League was renewed in 1198 and again in 1208. Not until 1226, however, when Frederick II reasserted imperial authority in northern Italy, did it again become a powerful factor in Italian politics for a long period. The new league was formed for 25 years by Milan, Bologna, Brescia, Mantua, Padua, Vicenza, and Treviso. They were soon joined by Piacenza, Verona, Lodi, and other cities, as well as by Boniface II of Montferrat and Godfrey of Biandrate. They received the support of the papacy and effectively opposed Frederick's reorganization of northern Italy. The league passed out of existence after Frederick's death in 1250.