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.