Initially a revolt by local Catholics against their Protestant overlords, many regional powers became involved as it potentially affected control of the Valtellina alpine passes.

A loose alliance of France, Venice, and the Duchy of Savoy supported the Three Leagues against the Grison rebels, backed by the Habsburg monarchy.

[2] At the same time, the Protestant and even a few Catholic representatives turned their ire on the Bishop of Chur, Giovanni or Johann V of Flugi.

[3] In 1618, the young radical Jörg Jenatsch became a member of the court of 'clerical overseers' and a leader of the anti-Habsburg faction.

With the support of the anti-Habsburg court, armed mobs attacked and arrested several pro-Habsburg leaders including the archpriest Nicholas Rusca and the provost at Johann Baptista who was known as Zambra.

[2] The harsh judgments of the Thusis court against a number of Catholics in the Valtellina led to a conspiracy to drive the Protestants out of this southern valley.

The leader of the conspiracy, Giacomo Robustelli of the Planta family, had ties to Madrid, Rome, and Paris.

On the evening of 18/19 July 1620, a force of Valtellina rebels supported by Imperial and Spanish troops marched into Tirano and began killing Protestants.

Early in the morning on 25 February, the troops attacked the castle and killed Pompeius Planta as he attempted to flee.

[5] Emboldened by the murder of Pompeius Planta, the Protestant forces in the Three Leagues assembled an army to retake the Valtellina and other subject lands.

The Catholic central Swiss cantons intervened under the abbot of Disentis on the side of the Spaniards and Austrians.

On 24 April 1622, the leader of the Capuchin mission, Fidelis of Sigmaringen, was murdered by Protestant peasants while on his way from Sewis to Grüsch near Chur.

Jürg Jenatsch and Ulysses von Salis used French money to hire an 8,000 man mercenary army and drive out the Austrians.

The peace treaty of Monzon (5 March 1626) between France and Spain confirmed the political and religious independence of the Valtellina.

Treaties with Austria in 1649 and 1652, brought the Müstair and Lower Engadine valleys back under the authority of the Three Leagues.