The British Expeditionary Force (BEF) was the British Army sent to the Western Front during the First World War. Planning for a British Expeditionary Force began with the Haldane reforms of the British Army carried out by the Secretary of State for War Richard Haldane following the Second Boer War (1899–1902).
The term "British Expeditionary Force" is often used to refer only to the forces present in France prior to the end of the First Battle of Ypres on 22 November 1914. By the end of 1914—after the battles of Mons, Le Cateau, the Aisne and Ypres—the old Regular Army had been wiped out, although it managed to help stop the German advance. An alternative endpoint of the BEF was 26 December 1914, when it was divided into the First and Second Armies (a Third, Fourth and Fifth being created later in the war). B.E.F. remained the official name of the British armies in France and Flanders throughout the First World War.
Emperor Wilhelm II of Germany, who was famously dismissive of the BEF, allegedly issued an order on 19 August 1914 to "exterminate ... the treacherous English and walk over General French's contemptible little army". Hence, in later years, the survivors of the regular army dubbed themselves "The Old Contemptibles". No evidence of any such order being issued by the Kaiser has ever been found.
Under the terms of the Entente Cordiale, the British Army's role in a European war was to embark soldiers of the British Expeditionary Force, which consisted of six infantry divisions and five cavalry brigades that were arranged into I Corps and II Corps. In October 1914, 7th Division arrived in France, forming the basis of III Corps and the cavalry had grown to form the Cavalry Corps of three divisions. By December 1914, the BEF had expanded to such an extent that the First Army and the Second Army were formed.
By the end of 1914, after the battles of Mons, Le Cateau, the Aisne and Ypres, the old regular British Army had suffered massive casualties and lost most of its fighting strength but had managed to help stop the German advance.
The force was commanded by Field Marshal Sir John French until December 1915, when he was replaced by General Sir Douglas Haig. The BEF's Chief of Staff on mobilisation was General Archibald Murray. He was replaced in January 1915 by General William Robertson. Lieutenant-General Launcelot Kiggell then served as Chief of Staff from December 1915 to January 1917 when he was succeeded by Lieutenant-General Herbert Lawrence. The two initial Army Corps were commanded by Douglas Haig (I Corps) and Horace Smith-Dorrien (II Corps).