From the military point of view, however, the important features of all this part of France are its roads and rivers, and most of all its rivers.
The three main waterways, the Oise, the Marne, and the Seine, converge as they approach Paris. Between the Oise and the Marne flows the main tributary of the Oise, the Aisne. Also north of the Marne is its tributary, the Ourcq; south of the Marne flows its tributaries, the Petit Morin and the Grand Morin. All join the Marne in the lower part of the valley not far from Paris. Between the Marne and the Seine flows the Aube, a tributary of the Seine. The country between the Marne and the Seine forms a wide swell of land. It was along the plateaux forming the backbone of this broad ridge that the Battle of the Marne was, for the most part, fought.
That brings us to the question of the roads.
Eastward from Paris, along the valley of the Marne, run three great highways. The most northerly, passing through Meaux, La Ferté-sous-Jouarre, Château Thierry, and Epernay to Chalons, follows nearly the same course as the river, crossing it at several points to avoid bends. The next branches off at La Ferté-sous-Jouarre, and also runs to Chalons by way of Montmirail. The third, passing through La Ferté-Gaucher, Sezanne, Fère Champenoise, and Sommesons to Vitry-le-François, follows the backbone of country already alluded to. All these great roads lead farther east into Germany, the northerly and the middle roads to Metz and the valley of the Moselle, the third road to Nancy and Strasburg.
Now, it must be manifest to anybody that command of these routes, with command of the railways corresponding with them, meant mastery of the communications between Paris and the French forces holding the fortified frontier all the way from Toul to Verdun.
If, consequently, the invading forces could seize and hold these routes and railways, and, as a result, which would to all intents follow, could seize and hold the great main routes and the railways running eastward through the valley of the Seine from Paris to Belfort, the fortified frontier--the key to the whole situation--would in military phrase, be completely "turned." Its defence consequently would have to be abandoned.