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 magazines exploded in close succession in 1946, the fort was repaired and upgraded with two fully automatic 150mm gun turrets with a range of 24km. After reassessments by the Swiss Armed Forces of their requirements for fixed fortifications, Dailly was progressively closed down starting in 1995, with full closure at the end of 2003.Fortress complexThe Fort de Dailly is the largest part of a fortress complex extending the length of the Saint-Maurice valley, with coordinated fields of artillery fire and interlocking defenses. While most of these positions are detached from one another and are self-sufficient installations, Dailly and its neighbor Savatan are connected. The Fort de Savatan adjoins Dailly at a lower level, facing to the north,and is connected by an inclined funicular in the mountain. The forts at Dailly and Savatan were completed in 1894 and became the central elements of Fortress Saint-Maurice. They were progressively expanded and upgraded through the twentieth century. Over time, Dailly was equipped with an example of virtually every Swiss artillery piece and construction method, from rock-cut artillery galleries to revolving gun turrets.