After completing their occupation of Belgium on August 20, 1914, German forces moved quickly upon France with two armies. Although fighting between French and German forces had taken place in the region of Alsace-Lorraine in southeastern France, the first joint French-British encounters with Germany occurred near the town of Mons along the Franco-Belgian border on August 23, 1914.
As French and British armies tried to halt the advancing Germans, they found themselves under heavy fire from long-range German artillery. With the German troops still well outside the range of their own guns, the Allied Powers were quickly forced to retreat. The allied retreat continued for two full weeks, allowing the Germans to advance over 120 miles to the river Marne, on the outskirts of Paris. For the Germans, the advance was not an easy one. As they retreated, the French and British armies took every opportunity to fight back and to hold each piece of ground for as long as they could.
On September 4, the Allied retreat was halted. The exhausted and sleep-deprived German troops faced an Allied defense reinforced with fresh troops brought in from Paris. On September 5, a decisive battle began that lasted five days. More than a million troops fought on each side as the Allies made their stand, determined to prevent the fall of Paris.
As the Germans drove at Paris from the southeast, a gap emerged between the German First and Second armies, and British and French commanders seized the opportunity to split the German forces apart by moving into the gap. French reservists were even ferried in to fill the breach using streams of taxicabs. The Germans were never able to regroup.
On September 9, after four days of intense fighting, the German armies found themselves unable to maintain their position on the Marne and began to fall back. British and French forces pursued the Germans doggedly and were able to drive them back forty-five miles, all the way back to the river Aisne. At this point, the Germans managed to dig in successfully and hold their position, taking advantage of a shorter supply line. A deadlock ensued, with neither side able to budge the other. The western front that formed would remain centered near this position for the rest of the war.
The aborted German invasion of France, though just a month into the war, marked a major turning point. Although World War I continued for four more years, this first failed advance is often cited as the point when Germany lost the war it had entered with such confidence. Unable to conquer France outright, Germany became mired in a war on multiple fronts. The Schlieffen Plan, according to which Germany would have quickly attacked and defeated France before Russia could mobilize and attack Germany, had failed. German military leaders, failing to adapt their strategy to cope with the new situation, suddenly faced a long, drawn-out war on an entrenched front.