The Habsburg pressure had prompted the Three Leagues of the Grisons to sign a close military alliance with the Old Swiss Confederacy in 1497/98. A raid of Habsburg troops on the Benedictine Convent of Saint John at Müstair on January 20, 1499 had been the immediate cause for the outbreak of the Swabian War, opposing the Swiss Confederacy and the Leagues of the Grisons against the forces of Maximilian, in particular the Swabian League. Part of Maximilian's strategic objective in this war was the control of the Umbrail Pass. Until April 1499, a large Habsburg army with troops from the Swabian League, from Tyrol, and mercenaries from Italy assembled at the villages of Mals and Glurns in the Vinschgau. At Calven, they erected strongly armed wooden fortifications (a so-called Letzi) blocking the entire exit of the Val Müstair. Habsburg troops tried to conquer the Pass dal Fuorn on May 11, 1499, but failed.