Germany’s plan in the event of war with both Russia and France in the beginning of the 20th century was to defeat France with the Schlieffen Plan and then concentrate on Russia. The Schlieffen Plan was named for the former German Chief of Staff Count Alfred Schlieffen. The idea was to let the French advance in the south and then seize Paris unexpectedly from behind from along the Channel coast. First, German armies on the left in the south would fix French forces in Alsace/Lorraine and the Saar, and even allow them to advance. Using this as a hinge, the Germans on the right in the north would swing like a door through Belgium, then along the channel coast, then finally down around the concentration of French forces and seize Paris from behind. On his death bed in 1913, just before the First World War, Schlieffen’s last words were, “Keep the right wing strong!” (The attack through Belgium and along the Channel coast.)
Unfortunately for Germany, the egos of the various German commanders couldn’t accept their roles. The prestigious commands were obviously on the right (those that were to seize Paris). These went to two very competent, but not very ambitious commanders: Generals Karl Von Buelow and Alexander Von Kluck. The commander on the left wing, i.e. the one who was supposed to let the French advance so they would be encircled by the right wing, was a very ambitious and out spoken Erick Von Falkynhahn. Finally, the commander in East Prussia, the stately Paul Von Hindenburg who was pulled out of retirement for the job of facing the Russians, also had an outsized influence on the Schlieffen Plan.
When the war started, the Russians mobilized much more quickly than expected and the proud Hindenburg refused to abandon East Prussia. So he essentially bullied the Chief of Staff, Helmuth Von Moltke the Younger (the Elder was his uncle who won the Franco Prussian war in 1870) for more forces. Naturally, they needed to come from Falkynhahn for the Schlieffen Plan to work. But Von Moltke was not his uncle. At the mere suggestion of giving up troops, Von Falkynhahn threw a fit, so Von Moltke the Younger took them from Buelow on the right wing, Moreover, Von Falkynhahn couldn’t contemplate the possibility of letting the French advance into his territory: It would look like he was losing in the newspapers. So instead of defending as per the Schlieffen Plan, he attacked… and kept attacking… and kept winning… and winning some more. Von Falkynhahn insisted that Von Moltke reinforce success, not Von Buelow who couldn’t even reach the sea without over extending himself (thanks to Hindenburg). More importantly though, Falkenhahn’s success pushed the French back – towards Paris.
Despite Schlieffen’s dying words, the German right wing was so weak that in the beginning of September, 1914, instead of attacking Paris from behind (north), Von Buelow and Von Kluck could only attack it from the front (east). Von Moltke still thought this would be good enough to seize Paris, except that Falkynhahn was too successful. Von Falkynhahn had basically bulled his way through the horrible terrain of the Ardennes forest, and was now spent. The French facing him were then in a perfect position to be sent to face Von Kluck and Von Bulow, a short cab ride away.
On 5 September 1914, the French commandeered 600 Parisian taxi cabs in a desperate attempt to move troops to the front along the Marne River in order to save Paris from the Germans. In actuality, only about 6000 French soldiers were ferried to the front in cabs, but afterwards hundreds of thousands would claim it. For the next week, more than one million British and French fought 1.5 million Germans to a standstill in the First Battle of the Marne. By 12 September, the German advance was stopped and Paris was saved. Over the next month, the front was solidified, and millions of soldiers dug their trenches. The war of maneuver was over , and the war of attrition began. The front line, which extended from the North Sea to Switzerland, wouldn’t change significantly for another four years.