The Great Retreat, also known as the retreat from Mons, was the long withdrawal to the River Marne in August and September 1914 by the British Expeditionary Force (BEF) and the French Fifth Army. The Franco-British forces on the Western Front in the First World War had been defeated by the armies of the German Empire at the Battle of Charleroi and the Battle of Mons. A counter-offensive by the Fifth Army, with some assistance from the BEF, at the First Battle of Guise failed to end the German advance and the retreat continued over the Marne. From 5 to 12 September, the First Battle of the Marne ended the Allied retreat and forced the German armies to retire towards the Aisne River and to fight the First Battle of the Aisne (13–28 September). Reciprocal attempts to outflank the opposing armies to the north known as the Race to the Sea followed from (17 September to 17 October).

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 250 km (160 mi) southward. The French and British halted their retreat in the Marne River valley while the Germans advanced to 40 km (25 mi) from Paris.