At the outbreak of the First World War in July 1914 Montgomery was appointed a general staff officer to the British Expeditionary Force (BEF) in France. He was appointed Chief of Staff at IV Corps in France in October 1914. Promoted to lieutenant colonel on 16 May 1915, he became Chief of Staff of Fourth Army of the BEF in February 1916, a role which, according to Field Marshal Sir Douglas Haig, from the planning for the Battle of the Somme in 1916 he carried out with "great ability and success". Promoted to the substantive rank of major general on 1 January 1917, he was appointed a Companion of the Order of the Bath for his services in the field on 1 January 1918. He was effectively Deputy Commander of the Fourth Army (deputising for General Sir Henry Rawlinson) in the final months of the War and played an important role in the success of the Battle of Amiens. He was appointed a Knight Commander of the Order of St Michael and St George for his services in connection with military operations in France and Flanders on 1 January 1919 and was also awarded the American Distinguished Service Medal by the President of the United States on 12 July 1919.