The Battle of Mons was the first action seen by the British Expeditionary Force during the First World War.
The British had aimed to hold the line at the Mons–Condé Canal against the advancing German First Army. Instead, the battle saw the BEF in a protracted retreat which lasted two weeks.
The Germans heavily outnumbered them, whilst the French Fifth Army’s own retreat saw the British right flank exposed.
After a fortnight of falling back, the British and the French were able to rally and push the Germans back at the Battle of the Marne.
In the history of the BEF, Mons is remembered for the disproportionate amount of casualties they were able to inflict on a numerically superior foe.
You can read more about the Battle of Mons here.