World War I
The British Expeditionary Force had been formed following the Boer War in case Britain was ever to take part in an overseas war. On the outbreak of World War I, it was sent to Belgium under the command of General Sir John French. The BEF was composed of four regular infantry divisions, rising later to seven infantry and three cavalry divisions. It sufferred defeat in its first battle at Mons, but helped to halt the German advance at the First Battle of the Marne.
In December 1914, the British Expeditionary Force was divided into the First and Second Armies, with a third and fourth created later in the war. Although the term 'British Expeditionary Force' strictly refers only to the forces sent initially to France in 1914, the name is often used of the British Army in France and Flanders throughout the First World War.
World War II
Following the German invasion of Poland the British Expeditionary Force was sent to the Franco-Belgian border in 1939. By May 1940, when German attacks began, it consisted of 10 infantry divisions in 3 corps, a tank brigade and a RAF detachment of about 500 aircraft. Commanded by Field Marshal Lord John Gort it sustained heavy losses during the German advance, and the remains were evacuated from in June, leaving much of their equipment behind.