Smith-Dorrien held senior commands in the British Expeditionary Force (BEF) during the First World War. He commanded II Corps at the Battle of Mons, the first major action fought by the BEF, and the Battle of Le Cateau, where he fought a vigorous and successful defensive action contrary to the wishes of the Commander-in-Chief Sir John French, with whom he had had a personality clash dating back some years. In the spring of 1915 he commanded the Second Army at the Second Battle of Ypres. He was relieved of command by French for requesting permission to retreat from the Ypres Salient to a more defensible position.