On September 5, 1914, after retreating for hundreds of miles, the British Expeditionary Force and the French Army turned and attacked the advancing German army.  The Germans had marched through Belgium and swept the British and the French before them.  They were pretty close to the end of their rope at that point, but their goals of surrounding their opponents and even taking Paris were tantalizingly close. Instead, the Germans drifted apart at a critical juncture between two armies, and the Entente forces, as Churchill said, “probed its way into the German liver”.  The Entente counterattacked along the Marne, and all hopes for German victory were lost.  After the battle, in which the French threw everything they had into turning the Germans back, or at least stopping them short of Paris, the Germans retreated back to what became the static front line that is the principle symbol of the war.

So what happened?  The Germans seemed to have the war in the bag, and then a miracle happened for the British and French.  One day the BEF was making plans to withdraw back to England, and a week later they’ve got the Germans on the run back to the Aisne.

Looking at the sources, there are a lot of opinions as to what caused the Germans to get caught in a bad position and forced to retreat.  Like a lot of situations, I believe it was a lot of different factors that led to the Allied victory at the Marne.  A lot of them boil down to the German’s not following their overall war strategy, the Schlieffen Plan.

First, Von Kluck’s forces were brought much further inland than the original plan called for (“letting the last man on the right, brush the Channel with his sleeve”).  If he had taken his drive further west, he would have easily outflanked the BEF and would not have had to fight as hard as he did to get within sight of Paris.

Second, the Germans counterattacked against French forces in the south-east portion of the Western Front.  Here, the French had attacked into Alsace-Lorraine and beyond, and after a few days were strung out and vulnerable.  The Germans took advantage of this, and drove them back.  However, the tactical victory contributed to a strategic defeat.  The French armies’ retreat back into France shortened both their supply lines and made units available for the French commander, Joffre, to use as he needed them in other parts of the front.  If the French had been allowed to continue their advance, even into Germany herself, those forces would not have been available to stem the flow of Germans in from Belgium, would have required supplies that the French needed to defend Paris, and would have been an easy fruit to either pluck or let wither on the vine after French collapse.

Finally, and this is debatable as to whether it mattered or not, the forces in the west were reduced by a small amount to provide unneeded reinforcements on the Eastern Front.  It was only a few corps that would have been in reserve, but I believe that had they been allowed to stay with their original war assignments, they could have relieved some of the worn-down German units that were pushed back at the Marne.  That may have made a difference in the final equation.

Basically, the Germans took their eyes off of the prize.  The Schlieffen plan was a judo move involving millions of men.  If the French had been allowed to overextend themselves while von Kluck made a buttonhook to get around the left flank of the BEF and the French, it would have made it very difficult for the French to not sue for peace as they had 40 years earlier.  By changing the plan, violating its precepts, and weakening their striking force, the Germans opened themselves up for eventual defeat.  It is interesting that after the Marne, there is rarely any talk on the German side of winning the war on the Western Front in the same way as it was envisioned prior to the first bicyclist crossing into Belgium.