Rhine and occupied Sundgau. They, in turn, were followed by the Franks following their victory at the Battle of Tolbiac in 496. Sundgau was incorporated into the kingdom of Austrasia and Christianity was introduced under the Merovingians. Francia About 750, the Duchy of Alsace was divided into two counties, Nordgau (Unterelsass) and Sundgau, the latter being mentioned in the Treaty of Mersen in 870. Historically then, Sundgau coincides with the lands of the counts of Ferrette and Habsburg, excepting the town of Mulhouse and its territories of Illzach and . Geographically, Sundgau denotes a more restricted area comprising the hilly country to the south of Mulhouse and reaching to the valley of Lucelle. During the 9th century and the 10th century Sundgau was administered by the Lieutfried family. Following the breakup of Charlemagne's empire, the region entered a period of instability, culminating in the emergence of feudalism. From 925 on, the Sundgau belonged to the Duchy of Swabia; it remained a part of Swabia until the Duchy disintegrated in the 13th century. Holy Roman Empire In 1125, Frederic, son of Theodoric I of Montbéliard, inherited the south of Alsace and became count of Ferrette. So, from 1125 to 1324, a large part of the Sundgau was administered by the counts of Ferrette. Ulrich III (1310–1324) conquered the valley of Saint-Amarin but died with no male issue. His daughter Jeanne married Albert II, Duke of Austria in 1324, and the County of Ferrette fell to Austria and was integrated with the other Habsburg possessions in the area. The Landgraviate of Sundgau (also known as Landgraviate of Upper Alsace), the successor of the Carolingian county, had been administered by the counts of Habsburg since 1135. They had owned the adjacent County of Sundgau even earlier. The Habsburgs enlarged their possessions in the area with numerous acquisitions in the following centuries, until by the mid-14th century almost all of the former Carolingian county was in the possession of Habsburg. Their consolidated territories in the area became known simply as the Sundgau, and belonged to the Austrian Circle of the Empire after 1512. The Habsburgian Sundgau was administered from Ensisheim by a bailli (bailiff or seneschal) and divided into four bailiwicks (Landser, Thann, Altkirch and Ferrette). Enguerrand VII, Lord of Coucy tried unsuccessfully to claim the Sundgau during the Gugler War of 1375. As of 1500, the Austrian Sundgau encompassed most of the southern