The First Battle of the Marne was a battle of the First World War fought from 5 to 12 September 1914. The German army invaded France with a plan for winning the war in 40 days by occupying Paris and destroying the French and British armies (Allies/Entente). The Germans had initial successes in August. They were victorious in the Battles of Mons and the Frontiers and overran a large area of northern France and Belgium. In what is called the Great Retreat the Germans pursued the retreating Franco/British forces more than southward. The French and British halted their retreat in the Marne River valley while the Germans advanced to from downtown Paris.
With the battlefield reverses of August, Field Marshal John French, commander of the British Expeditionary Force (BEF), lost faith in his French allies and began to plan for a British retreat to port cities on the English Channel for an evacuation to Britain. However, the French commander Joseph Joffre maintained good order in his retreating army and was able to reinforce it by bringing in additional manpower from his eastern flank and integrating military reserve units into the regular army. By early September, the Franco–British forces outnumbered the Germans who were exhausted after a month-long campaign, had outrun their supply lines, and were suffering shortages. On 3 September the military governor of Paris, Joseph Simon Gallieni, perceived that the German right flank was vulnerable and positioned his forces to attack.
On 4 September Joffre gave the order to launch a counteroffensive. The battle took place between Paris and Verdun, a west to east distance of . The point of decision and the heaviest fighting was in the western one-half of that area. By 9 September, the success of the Franco–British counteroffensive left the German 1st and 2nd Armies at risk of encirclement, and they were ordered to retreat to the Aisne River. The retreating armies were pursued by the French and British. The German armies ceased their retreat after on a line north of the Aisne River, where they dug in on the heights and fought the First Battle of the Aisne. The German retreat from 9 to 12 September marked the end of the German attempt to defeat France quickly. Both sides next commenced reciprocal operations to envelop the northern flank of their opponent in what became known as the Race to the Sea which culminated in the First Battle of Ypres and led to a bloody four-year long stalemate of trench warfare on the Western Front of World War I.
The Battle of the Marne from September 5 to 12 resulted in an estimated 250,000 French casualties, 12,733 British casualties and 298,000 German casualties. Holger Herwig called the Battle of the Marne the most important land battle of the 20th century. The battle is described in French folklore as the "miracle on the Marne."
Background
The Combatants
Germany and France declared war on each other on 3 August 1914. Both sides expected a short war. Germany faced the specter of a two-front war, facing Russia in the east and France and Britain in the west. German strategy was to defeat France in six weeks and then turn its attention to Russia. As envisioned by the Schlieffen Plan (revised by Helmuth von Moltke), the Germans embarked on a rapid, circular, counter-clockwise offensive through Belgium and into France with the objective of capturing Paris and enveloping and destroying the French army east of Paris within their six-week timetable. The German violation of Belgium's neutrality brought Great Britain into the war. Britain sent the British Expeditionary Force (BEF) to France.