The Brittany War refers to a series of conflicts in the late 14th century in the region of Brittany, located in present-day France. The war primarily involved the struggle for control over the duchy between the rival factions of the House of Montfort and the House of Blois. This conflict was part of the larger context of the Hundred Years' War and was marked by shifting alliances and battles. The war began in 1341 with the death of Duke John III of Brittany, leading to a succession crisis. The conflict saw significant battles, including the Battle of Auray in 1364, which ultimately resulted in the victory of the Montfort faction. The war concluded with the Treaty of Guérande in 1381, solidifying the Montfort family's control over Brittany.