The dispute over the Ten Towns League not only captivated Alsace but also the Holy Roman Empire of the German Nation, France, and Europe in the second half of the 17th century. The starting point, pivot, and focal point were unclear provisions in the Peace of Westphalia, which granted France, among other things, sovereign possession of the imperial landvogtei of Hagenau. Due to the close ties between the towns' league and the landvogtei, the question arose regarding the changed status of the ten imperial cities. After a fundamental introduction to the structure and organization of the Decapolis, this work focuses on the multifaceted political, diplomatic, and military conflicts surrounding the Alsatian league cities. The basis is a wide range of previously unexplored sources in Alsace, Paris, and Vienna. In addition to high politics, these documents repeatedly provide insights into everyday life along the Upper Rhine.