The First Battle of the Marne was a battle of the First World War fought from 5 to 12 September 1914. It was fought in a collection of skirmishes around the Marne River Valley. It resulted in an Entente victory against the German armies in the west. The battle was the culmination of the Retreat from Mons and pursuit of the Franco–British armies which followed the Battle of the Frontiers in August and reached the eastern outskirts of Paris.

Field Marshal Sir John French, commander of the British Expeditionary Force (BEF), began to plan for a full British retreat to port cities on the English Channel for an immediate evacuation. The military governor of Paris, Joseph Simon Gallieni, wanted the Franco–British units to counter-attack the Germans along the Marne River and halt the German advance. Entente reserves would restore the ranks and attack the German flanks. On 5 September, the counter-offensive by six French armies and the British Expeditionary Force (BEF) began.

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 40 mi (65 km) 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 13 September marked the end of the attempt to defeat France by crushing the French armies with an invasion from the north through Belgium and in the south over the common border. Both sides 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.