The Fort de Dailly is the largest component of Fortress Saint-Maurice, which is in turn one of the three principal fortified regions of the National Redoubt of Switzerland. Almost entirely subterranean, the Fort de Dailly was built in the Massif de Dailly to the east of Saint-Maurice beginning in 1892. With the Fort de Savatan, it comprises a fortress complex encased in rock high above the strategic Saint-Maurice valley, capable of commanding the valley from Chillon to Martigny. It is the central component of Fortress Saint-Maurice in terms of both its position and its military power. Construction began in 1892 and the fort became operational in 1894. The fort was nearly constantly upgraded with new artillery in increasingly secure positions. Following an incident in which three ammunition