The Milanese reeve of Como, Gian Giacomo Medici, who resided in castle of Musso, had been raiding the valley of Chiavenna since 1521.
Due to religious conflicts following the Reformation, neither the Swiss Confederation nor the Three Leagues were able to move quickly to free the prisoners.
In the spring of 1526 a Three Leagues delegation, with the support of France and Venice, were finally able to secure the release of the prisoners, but only after paying a ransom and giving the Tre Pievi or three communes at the north end of Lake Como to Medici.
[2] Due to the weak 1526 treaty, in 1531 Milan attacked again, capturing Morbegno and defeating the League forces in the area.
[2] In a peace treaty concluded the next year, Chiavenna and the Valtellina were granted to the Three Leagues; only the tre pievi came under the supervision of Milan.