Keir had hardly been in command of the 6th Division for a month when the World War I began, and it was mobilised as part of the British Expeditionary Force for service on the European Continent. However, the original plan of sending six divisions to France was altered due to fears of German landings in the United Kingdom, and the 6th Division spent the first month of the war in reserve in East Anglia. In September 1914 it landed in France, and immediately saw service at the Battle of the Aisne. Later in the year, he commanded it saw action at the Battle of Armentières. In mid-1915, Keir was appointed to take command of the newly formed VI Corps (United Kingdom), which took part in the Battle of Loos in September 1915. In December 1915 VI Corps was attacked by the Imperial German Army with phosgene gas, the this form of chemical warfare was used.