At the outbreak of the First World War, the regiment was mobilised and began its training in England. In 1915, Lord Strathcona's Horse served as infantry in the trenches in France. On 16 February 1916, the Strathcona's were reconstituted as a mounted force and, as an Imperial Service Regiment, served in the Canadian Cavalry Brigade attached to the 2nd Indian Cavalry Division, which in November 1916 became the 5th Cavalry Division of the British 4th Army. In March 1917, the regiment saw action as cavalry during the defence of the Somme front. It was during this fighting that Lieutenant Frederick Harvey won the Victoria Cross for rushing a German machine gun post and capturing the gun position. In spring 1918 during the last great German offensive, called by the Germans Operation Michael, when the Imperial and French armies were on the verge of being split, the regiment earned its third Victoria Cross.