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 150 mm gun turrets with a range of 24 kilometres (15 mi). 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.

The 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.

The Fort de Dailly and surrounding installations were manned through their modern history by Fortress Brigade 10. Dailly and Savatan comprised Fortress Group 2, under Fortress Regiment 19 of Fortress Brigade 10.

Fort de Dailly is one of the largest and most heavily armed forts in Switzerland, and the central position in Fortress Saint-Maurice. The artillery fort is located on the end of the Dailly massif at a higher level than Savatan, with fields of fire to the north, west and south. The fort therefore is capable of firing on an enemy advancing southeast into the heart of the Alpine redoubt from the direction of Lac Léman, or northwest from the Great Saint Bernard or Simplon passes. Work began at Dailly in 1892. The entrance to the underground fort lies at an elevation of 1,400 metres (4,600 ft) at the end of 29 switchbacks. The fort initially was armed with six 120 mm gun turrets with additional open artillery positions, including six additional 120 mm guns on rail-mobile disappearing mounts. Two 75 mm guns were mounted in casemates, while portable 53 mm gun turrets occupied prepared surface positions. Dailly was upgraded with 105 mm artillery during the 1940s, with as many as ten 105 mm guns in casemate positions. Flanking coverage from across the valley was provided by the Fort du Scex.

The construction of Dailly and other fortifications at Saint-Maurice raised concerns in France at the turn of the twentieth century. The Saint-Maurice defenses were first reconnoitered in December 1896 by a French Colonel Raymond, followed by Lieutenant Vignon in July, 1898.. French citizens working on the fortification projects were interviewed by the French gendarmerie. In 1901 the French military attaché in Bern obtained pictures of Dailly from a Swiss officer.

Initial studies were carried out for a fortification at Savatan in 1890. It became clear that a position in the higher Dailly massif to the south would protect forces on the Savatan plateau and would complement a battery on the opposite side of the valley at Scex. A small hotel on the site of an old weather observation station on the Dailly massif was chosen for the barracks. Robert Weber was the architect in charge of the works. Initial work installed two 150 mm mortars on Dailly at an altitude of about 3,000 metres (9,800 ft), which remained in place until the 1920s. A single 120 mm howitzer in an armored turret was installed on the summit, remaining in place through the 1940s. For local defense, two 84 mm guns were placed in concreted casemates near an observation point in 1894, called the Batterie de la Galerie de Morcles.

Heavier artillery was initially installed in open positions. 150 mm Krupp field guns were installed in open emplacements in 1894, allowing fire in two directions. Battery C15 was placed on the ridge between Righi and the Aiguille, firing north and south. Battery C10 with two 105 mm guns and the Batterie Aiguille with the same equipment were placed near the summit. The Batterie de l'Observatoire, located above the Galerie des Mordes, was equipped with four 120 mm guns. The Batterie Rossignol placed two 120 mm pieces each facing north and south, while the Batterie de Plan 2 and the Batterie de Righi each mounted two 120 mm pieces. These were replaced in 1923 by Model 1912 120 mm howitzers with a higher rate of fire.

As construction continued, rotating guns were installed to exploit the broad field of fire afforded by the heights. Four or six 120 mm guns on mobile were installed on tracks in the Righi area. The guns were stored in garages in the rock, and were in service until the 1930s, forming batteries E1, E2 and E3. Two or four mobile guns were converted to fixed positions, replacing the C10 battery. Fixed turret guns were installed in the 1930s. The Model 1939 105 mm gun was developed with an underground barbette, with the gun carriage in an armored turret and only the barrel exposed. Battery Panaux, with two turrets, was placed in service in 1940, remaining in service until 1994.