THE frontier between France and Germany is one of the oldest, as well as one of the newest, problems of European politics. It is, from one point of view, as old as Caesar's Gallic wars and the Germanic invasions of the Roman Empire, as new as the treaty of Versailles, and even newer. Some elements in it are almost secular in their persistence, some are only of recent origin. Moreover, the problem is much more complicated than is usually supposed, for we have here a frontier zone rather than a single line, a compound not a simple fracture; while the line which would be drawn in one place by one set of considerations would be drawn elsewhere by other forces.
The oldest historic frontier in this region is the Rhine, for nearly five centuries the boundary between Gaul and Germany, in speech and culture as well as in government. The Germanic invasions of the fifth century pushed the German element well to the westward, establishing the western limit of Germanic speech much where it lies today, and creating a mixed Gallo-German population in what is now western Germany as well as in northern and eastern France. At the same time the political frontier disappeared with the formation of a Frankish kingdom, and later a Frankish empire, ruling over both Gaul and the lands to the east of the Rhine, only to reappear in the partitions of the ninth century. The treaty of Verdun in 843 set apart the France and Germany of the future, separating them by a middle region of Lotharingia which was to be the bone of contention between them for more than a thousand years. With the partitions of Lotharingia the Franco-German problem begins to take on a more modern form. The best-known of these, the treaty of Meerssen in 870, gave to the German kingdom the whole left bank of the Rhine, including Strasbourg, Trier, Metz, and the upper valleys of the Meuse and Moselle, whence this treaty has