On September 6, 1914, some 30 miles northeast of Paris, the French 6th Army under the command of General Michel-Joseph Manoury attacks the right flank of the German 1st Army, beginning the decisive First Battle of the Marne at the end of the first month of World War I.

After invading neutral Belgium and advancing into northeastern France by the end of August 1914, German forces were nearing Paris, spurred on by punishing victories that forced five French armies into retreat after the Battles of the Frontiers at Lorraine, Ardennes, Charleroi and Mons. In anticipation of the German attack, the anxious French government appointed the 65-year-old General Joseph-Simon Gallieni as the military governor of Paris. Gallieni, predicting that the Germans would reach Paris by September 5, did not wish to sit idly back and wait for invasion. In the first days of September, he managed to convince the French commander in chief, Joseph Joffre, to spare him an army—Manoury’s 6th Army—from the front in order to aggressively defend the capital.

At the same time, General Alexander von Kluck, at the head of the German 1st Army, was disobeying orders from its own headquarters to double back and support General Karl von Bulow’s 2nd Army, thus protecting itself from possible attacks from the French on its right flank, from the direction of Paris. Not wanting to subordinate himself to Bulow’s command, Kluck ordered his forces to proceed in their pursuit of the retreating French 5th Army, under General Charles Lanrezac, across the Marne River, which they crossed on September 3. When Gallieni learned of Kluck’s move that morning, he knew the French 6th Army—the new army of Paris—had been given its opportunity to attack the German flank. Without hesitation, he began to coordinate the attack, urging Joffre to support it by resuming the general French offensive earlier than army headquarters had planned.

On September 4, Helmuth von Moltke, chief of the German general staff, learned that Kluck had disobeyed orders, and that his troops—exhausted and depleted of resources, having outrun their lines of supply over the course of their rapid advance—had crossed the Marne. Fearing the attack from Paris on the 1st Army’s exposed flank, Moltke ordered that the march of the 1st and 2nd Armies towards Paris be halted in order to face any threat from that direction. The order came too late, however, as Gallieni had already readied his army for an attack, and Joffre—with help from the British minister of war, Lord H. H. Kitchener—had obtained the promised support of the British Expeditionary Force (BEF), commanded by Sir John French, for the French 5th and 6th Armies in their renewed offensive against German forces at the Marne.

On the morning of September 6, the 150,000 soldiers of Manoury’s 6th Army attacked the right flank of the German 1st Army, whose turn to meet the attack opened a 30-mile-gap between Kluck’s forces and Bulow’s 2nd Army. Acting quickly, the French 5th Army—under a new leader, General Louis Franchet d’Esperey, appointed by Joffre to replace Lanrezac—and divisions of the BEF poured into the gap and simultaneously attacked the German 2nd Army. Fierce fighting continued over the next several days, with Manoury’s exhausted army managing to hold its ground only after being reinforced on September 7 by a corps of 6,000 rushed from Paris in taxi cabs. After Franchet d’Esperey’s 5th Army launched a successful surprise attack on the German 2nd Army, Moltke ordered a general German retreat on September 9. Over the next few days, Allies slowly pushed the Germans back towards the Aisne River, where the 1st and 2nd Armies dug in, beginning the entrenchment of positions that would last well into 1918.

The Allied check of the German advance during the Battle of the Marne made the struggle one of the most decisive battles in history. Events at the Marne signaled the demise of Germany’s aggressive two-front war strategy, known as the Schlieffen Plan; they also marked the end of the general belief, held on both sides of the line, that the conflict that broke out in the summer of 1914 would be a short one. As the historian Barbara Tuchman wrote as a conclusion to her book The Guns of August (1962): “The Battle of the Marne was one of the decisive battles of the world not because it determined that Germany would eventually lose or the Allies ultimately win the war but because it determined that the war would go on. There was no looking back, Joffre told the soldiers on the eve. Afterward there was no turning back. The nations were caught in a trap, a trap made during the first thirty days out of battles that failed to be decisive, a trap from which there was, and has been, no exit.”