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The Musso war was an armed conflict between the federation of the Three Leagues (Drei Bünde) in the Grisons, an associate state of the Old Swiss Confederacy, and the Duchy of Milan early in the 16th century.
The Milanese reeve of Como, Gian Giacomo Medici, who resided on castle Musso, had been raiding the valley of Chiavenna since 1521, and in 1526 he even occupied the Valtellina, both subject territories of the Three Leagues. The Three Leagues managed to drive the Milanese forces back. In 1531, when another Milanese attack on the Valtellina occurred, the Three Leagues called the Old Swiss Confederacy for help, calling in duties of a defence alliance concluded earlier. The confederacy was at that time already entrapped in religious conflicts following the Reformation, and only the Protestant cantons supported the Three Leagues. In a peace treaty concluded the next year, Chiavenna and the Valtellina were granted to the Three Leagues; only three communes at the north end of Lake Como, the tre pievi, came under the supervision of Milan.
The refusal of the Catholic cantons to support the Three Leagues in these skirmishes was taken by the Swiss canton of Zürich as the reason to start the second war of Kappel. The Catholic cantons would emerge victorious from that war and even gain the majority in the confederacy's federal assembly, the Tagsatzung, with far-reaching consequences for the confederacy.