Answer:
French and British forces launched an unexpected counterattack.
Step-by-step explanation:
The World at War: Western Front
Having made the move to attack both France and Russia, the Central Powers began fighting a two-front war. In the west, they faced France. In the east, they faced Russia. A German field marshal named Alfred von Schlieffen devised a plan to reduce the stress of fighting on two fronts simultaneously. Under the Schlieffen Plan, Germany would quickly take out the French and then turn troops' attention to Russia. The plan assumed that the Russians would be slow to mobilize. Germany believed it would take Russia weeks to reach the German border. The Germans also assumed the French would be weak and easily defeated. In the time it took Russia to mobilize, the Germans thought they could defeat the French on the western front.
The Germans managed to quickly advance through France according to plan. By September, they were within 30 miles of Paris. However, in the first major battle, the Battle of the Marne, France and Britain launched a counterattack to stop the German advances. For the three-day battle, the Germans were stopped and pushed back to the Aisne River. The defeat was a shock to the Germans, as they had clearly underestimated the strength of their opponents. The Schlieffen Plan was a failure. After the Germans realized that they could not win a quick victory, they were forced to send many of their soldiers from France to meet the Russians in the East.