Answer
Smith-Dorrien first fought the BEF at the Battle of Mons. The BEF was the British Expeditionary Force, which was sent to France to fight in World War I. The Battle of Mons was fought on August 23, 1914, and was the first major battle of World War I. The BEF was outnumbered by the German Army, but was able to hold off the Germans for several hours.