Treaties of Nijmegen
During the Late Middle Ages the Austrian Habsburg Dynasty acquired first the Sundgau in 1324 and then the County of Burgundy (more usually known in this period as the "Franche-Comté") in 1493. The Habsburgs were obliged to cede the Sundgau to the Kingdom of France under the 1648 Peace of Westphalia, and the Belfort Gap therefore became the border between French Alsace and Habsburg Franche-Comté, but not for long as France acquired Franche-Comté too under the 1678 Treaties of Nijmegen.