The Battle of Mons was the first major action of the British Expeditionary Force (BEF) in the First World War. It was a subsidiary action of the Battle of the Frontiers, in which the Allies clashed with Germany on the French borders. At Mons, the British Army attempted to hold the line of the Mons–Condé Canal against the advancing German 1st Army. Although the British fought well and inflicted disproportionate casualties on the numerically superior Germans, they were eventually forced to retreat due both to the greater strength of the Germans and the sudden retreat of the French Fifth Army, which exposed the British right flank. Though initially planned as a simple tactical withdrawal and executed in good order, the British retreat from Mons lasted for two weeks and took the BEF to the outskirts of Paris before it counter-attacked in concert with the French, at the Battle of the Marne.

The British Expeditionary Force (BEF) was the six divisions the British Army sent to the Western Front during the First World War. Planning for a British Expeditionary Force began with the 1906–1912 Haldane Reforms of the British Army carried out by the Secretary of State for War Richard Haldane following the Second Boer War (1899–1902).