The war of Milan against Como, sometimes called the 10-Year War, was a conflict over the control of Insubria, Brianza and Valtellina in the early 12th century, fought between the comuni of Milan and Como. It resulted in the defeat and destruction of the latter, cementing Milan's position as the dominant power in Lombardy.
Background
At the beginning of the 12th century the two comuni of Milan and Como had been rivals for many decades, especially for ecclesiastical and commercial disputes. Although Milan was a prominent trading power, the territory under its direct control was limited to the city itself and to a small strip of countryside beyond the walls. The Milanese merchants, therefore, had to pay heavy duties every time they went to trade in nearby cities. Como, in addition to being itself a thriving trading city and therefore a competitor, was located on the road that connected the Lombard capital to Switzerland through the passes of Lucomagno, Spluga and Settimo. Milan also aimed at freeing itself from the control of the Holy Roman emperor, and the possession of Lake Como and Canton Ticino could guarantee relative security in this sense, preventing any armed incursions from Germany. Como resented the fact that part of the eastern branch of the lake, the town of Lecco and the countryside of the Martesana, were under the influence of Milan. In fact, as early as the middle of the 11th century, the Milanese municipality had striven to gain autonomy from the Holy Roman Empire and, formally, Como depended on Milan. The political affirmation of Como could therefore only pass through an alliance with the Imperial government and with the reduction of the power of Milan.
A considerable role was also played by ecclesiastical matters. The Diocese of Como had dissolved its ties with the Archdiocese of Milan at the time of the tricapitoline schism in 607, becoming subordinated to the patriarchate of Aquileia. The bishop Ariberto da Intimiano, who died in 1045, had ruled the Milanese archdiocese from 1018 and had supported the claims of the emperor Henry IV against pope Gregory VII during the investiture controversy, up to the Humiliation of Canossa. Conversely, the bishop of Como Rainaldo had always maintained a pro-papal attitude, attracting the antipathies of most of the Lombard bishops and ending up imprisoned twice. Towards the end of 1083 he was released from his second imprisonment, served in Rome, but he had to renounce the bishopric and retired to Nesso, where he died the following year.
In 1094 the Emperor Henry IV of Franconia, who had claimed for himself the right to appoint bishops in dispute with Pope Gregory VII, appointed Ariberto (or Eriberto) de Amatia from Venosta, whose family were the feudal lords of Bormio. The new bishop died in 1096, and in his place the emperor appointed the deacon Landolfo da Carcano, who was consecrated by Federico di Moravia, Patriarch of Aquileia. Landolfo, hailing from a noble Milanese family, was considered an expression of the interests of his city and he was accused of simonia and not recognized as legitimate by the citizens and the clergy of Como. When Pope Urban II arrived in Como in the same year to consecrate the new cathedral of Sant'Abbondio, he canceled the imperial appointment by renewing the accusation of simonia to Landolfo, and called a new election. The clergymen of the Como Cathedral elected in his place Guido Grimoldi (or Guidone), a native of Cavallasca, archpriest of the Basilica di Sant'Abbondio and member of the local nobility.
In 1098 Landolfo was excommunicated by Anselmo da Bovisio, archbishop of Milan. Nonetheless, he refused to resign from his position, not wanting to lose the privileges and the religious and political power that the office of bishop entailed. Aware of lacking the strength to oppose Grimoldi and being unable to set foot in Como, he took refuge in the castle of San Giorgio in Magliaso, a small village near Lugano, then part of the diocese of Como. From that place he continued to act as he was the legitimate bishop. He granted investitures and privileges to ingratiate himself with the nobles of lakeside villages such as Samolaco, periodically organized raids against villages loyal to Como, and wrote several times to the new archbishop of Milan Giordano da Clivio, with whom he was related, asking for help in the reconquest of his seat in Como. The expansion of Landolfo's influence on the territory of the diocese of Como pushed the people of Como to resort to force. The diocese of Como therefore split into two factions: the first, headed by Guido Grimoldi, was mainly composed of cives, the citizens of Como; the second, led by Landolfo da Carcano, represented the inhabitants of the countryside.