On the outbreak of the First World War he had risen to the rank of lieutenant and was serving as a platoon commander in A Company under Major Lord George Stewart-Murray. The battalion sailed to France in August 1914, where Fortune saw initial action during The Retreat from Mons and the First Battle of the Marne. In September 1914 he was promoted to captain, taking command of A Company following the death of Major Lord Stewart-Murray at the First Battle of the Aisne. Fortune led A Company ably through The First Battle of Ypres before moving up to battalion headquarters as adjutant on 11 November 1914, where he saw further action at Givenchy, Cuinchy, Neuve Chapelle and Aubers Ridge.
The 51st Division remained in France after the general evacuation from Dunkirk, having been assigned to the French IX Corps. After naval evacuation proved impossible and supplies of ammunition had been exhausted, Major-General Fortune was forced to surrender the greater part of the Highland Division at St Valery en Caux.[6]
Fortune spent the rest of the war as a prisoner of war. As a senior British officer in captivity in Germany, he worked to improve the conditions of the men under his command. He suffered a stroke in 1944 but refused repatriation.[2] He was finally liberated in April 1945 and made KBE shortly after.[2][5]
Several British writers have questioned the decision to remain with the French during the battle.[citation needed] However, General Charles de Gaulle stated, 'For my part, I can say that the comradeship of arms, sealed on the battlefield of Abbeville in May–June 1940, between the French armoured division, which I had the honour to command, and the gallant 51st Scottish Division under General Fortune, played its part in the decision which I made to continue the fight at the side of the Allies, to the end, come what may'. And he concluded by quoting the old motto of the Garde Ecossaise: omni modo fidelis – faithful in every way.[7]