The Battle of Mons was the British Expeditionary Force's first major combat of the First World War. The BEF had advanced into Belgium on the left of the French Fifth Army and took up position on a 20 mile front along the Mons-Condé Canal on August 22. When the Fifth Army was defeated in the Battle of Charleroi , the BEF commander, General Sir John French, agreed to hold his position for 24 hours. On the morning of August 23, General Alexander von Kluck's German First Army, advancing on the right of the German line, encountered the British forces.

The BEF comprised four regular army divisions arranged in two corps; I Corps and II Corps. The British infantry were experienced, professional soldiers. They had prepared hurried but deep defensive positions and were capable of producing rapid, accurate rifle fire at 15 rounds a minute. The German battalions advanced against the British line in "parade ground formation" and were decimated. So intense and continuous was the shooting that the Germans believed they were facing machine guns but at the time the British had only two machine guns per battalion—nearly all the damage was done by riflemen.

The British suffered 1,600 casualties but morale remained high and the troops believed they could continue to hold off the German advance. However, with the French in retreat on the right, the BEF's position was untenable and a general retreat began on the morning of August 24. The retreat would continue for 14 days, taking the BEF all the way to the outskirts of Paris.

Accounts of the battle and the retreat in the newspapers resulted in a rapid rise in army recruitment in Britain. By spring of April 1915 rumours were circulating which claimed the intervention of a miracle, the Angels of Mons which aided British troops.