Because all of France’s military power was arranged to challenge any German advance over their (France’s) eastern border. Germany needed a swift victory over the French in order to meet the timetable of the Schlieffen Plan, the strategic plan to defeat France and pivot German forces to the east to face the slow mobilising Russians. French forts along the border would have held up the German advance, giving the Russians the time they needed to mobilise and advance into eastern Germany.

France also had plans to March straight into Alsace-Lorraine when a war began. This would have led to the region becoming a major battlefield if the Germans were also advancing through the region, again, this would have had major ramifications for the timetable dictated by the schlieffen plan