Requires some very deep PoDs in the 19th Century to get to this point.
I would point out the Germans did have some effective fortifications on the western frontier. Those at Metz were extensive & well built. They were also made economical by routing the high capacity railways between France and Germany through the region covered by this group or system of fortifications. It was not necessary to cover the entire frontier. Another set of forts covering Strausberg added depth to a western front defense. If a French army is to advance north into the Rhineland towards the Ruhr, or cross the Rhine & head east securing Metz and Strasberg is essential, otherwise the French lack effective railway transport and are trying to supply modern 20th Century armies with horse wagons.
Sending a left wing on a flanking maneuver to the north around the Metz fortress complex finds a few low capacity railways in the Ardennes. Like the Germans the French must swiftly eliminate the Namur and Liege fortification groups and restore the high capacity railways that routed through those regions. To resolve the fortification problem the French must either invest in heavy siege artillery, appropriate ammunition, and artillery tactics. Or develop some sort of system of Sappers/Pioneers and tactics that will rapidly defeat these fortress systems.