The modern Chablais is a area that covers parts of Eastern France and Western Switzerland. In Switzerland, this French-speaking region lies between the Rhone and Lake Geneva. The Swiss area extends over the cantons of Valais and Vaud.
In France, this region is located in the department of Haute-Savoie and is bordered to the north by Lake Geneva and the Swiss border town of St. Gingolph. At the time of the conquest by Romans in the first century B.C., the area was inhabited by the Celtic tribe of the Nantuates, who soon romanized.
Chablais became part of the Roman province of Raetia-Vindelica and from the end of the second century the newly formed province of Alpes Poeninae. After the fall of the Roman Empire in 476 A.D., it briefly became part of the (first) Burgundian kingdom (around 461-534), after which it merged into the Frankish Empire of the Merovingians and later Carolingians. After the division of the Carolingian Empire in 843 at the treaty of Verdun, the area became part of the (second) Burgundian kingdom (888-1032).
After 1032 it formally became part of the Holy Roman Empire, but local rulers had the upper hand. The numerous castles still bear witness to this.
Then, from the thirteenth century onwards, the centuries of political and military struggle for hegemony began between the dukes of Savoy, the bishop of Sion and the city of Bern. Savoy would be the big loser. In 1475, the bishop of Sion conquered the Chablais to Massongex.
In 1476, with the support of the bishop of Sion, Bern conquered the remaining part of the Chablais, including the region around Bex. In 1536 Bern even conquered the French part of Chablais, including Thonon and Evian. By the treaties of Lausanne (1564) and Thonon (1569) borders were established as they still are today.
The river Morge near St. Gingolph became the border between the French and Swiss Chablais (i.e. the Eidgenossen), Evian and Thonon fell under the duke of Savoy again. It was only centuries later, with and after the French invasion in 1798, that the cantons of Valais and Vaud came into being.
The Swiss territory of the Chablais would not change anymore. The towns of Aigle, Villeneuve, Yvorne, Massongex, Ollon, Bex and Saint-Maurice are silent witnesses of this history.