Why did Germany’s plan to win in six weeks fail at the Marne?

Germany's plan to win quickly failed because the German armies could not move and be supplied fast enough, and at the Marne in September 1914 the French and British counter-attacked and forced them back. Germany feared a war on two fronts, so its strategy was to defeat France in six weeks and then turn to Russia. The famous Schlieffen Plan was a way to do it: a rapid, counter-clockwise offensive through Belgium to capture Paris and destroy the French army, inside a six-week timetable.

At first it seemed to work. In what is called the Great Retreat the Germans pushed the French and British back more than 250 kilometres, to within 40 kilometres of Paris. But by early September the Franco-British forces outnumbered the exhausted Germans, who had outrun their supply lines. Joseph Gallieni, the military governor of Paris, saw that the German right flank was vulnerable. On 4 September Joseph Joffre, the French commander, ordered a counter-offensive.

By 9 September two German armies risked encirclement and were ordered to retreat to the Aisne River. The retreat from 9 to 12 September ended the German attempt to defeat France quickly. The estimated casualties of the battle were 250,000 French, 12,733 British and 298,000 Germans. The German violation of Belgium's neutrality had also brought the United Kingdom into the war.

Historians still argue about the plan itself. What is clear is what followed: each side tried to go around the other's northern flank in the Race to the Sea, which led to four years of trench stalemate on the Western Front.