How did the First Battle of the Marne stop Germany's advance toward Paris?

The First Battle of the Marne (September 6–12, 1914) halted the German advance toward Paris by exploiting a critical vulnerability in the German line created by the rapid, disjointed movement of their forces.123

As the German First Army under General Alexander von Kluck wheeled inward east of Paris to pursue retreating Allied forces, it exposed its right flank. Recognizing this, General Joseph-Simon Gallieni, the military governor of Paris, urged an immediate counterattack. French commander Joseph-Jacques-Césaire Joffre ordered a general offensive, with the French Sixth Army striking the exposed German flank.24

This pressure forced Kluck to divert his troops to protect his flank, creating a 30-mile gap between the German First and Second armies. Allied forces, including the British Expeditionary Force, exploited this gap, compelling the German armies to retreat to the Aisne River. The German failure was exacerbated by the exhaustion of their troops, who had outrun their supply lines, and the resilience of the French army, which successfully transitioned from a retreat to a decisive counteroffensive.24