The confederation attempted to expand into the southern foothills of the Alps to gain control of both ends of the valuable mountain passes.

Uri was quickly joined by forces from other Confederation cantons and established a siege camp below the walls of Bellinzona on 30 November 1478.

The Duke of Milan responded by sending 10,000 men toward Bellinzona to drive the Confederates back and reassert his control over the Leventina.

The defenders were able to defeat the much larger force because the Milanese army was confined in a narrow valley, struggling for foothold on the December snow and ice.

Against this attack, the Milanese army was helpless regardless of its superior number, and they were forced to flee, leaving an estimated 1,400 dead.