Upon the commencement of the Second World War in September 1939, Hargest attempted to join the Second New Zealand Expeditionary Force being raised for service. His application was initially declined for health reasons, but after intervention by Peter Fraser, the acting Prime Minister of New Zealand, he was accepted and appointed commander of the 5th Infantry Brigade, part of the 2nd New Zealand Division. He led his brigade during the Battle of Greece in April 1941 after an initial period performing garrison duty in England. During the Battle of Crete he displayed poor judgement in positioning his forces around the vital Maleme airfield and in controlling their movements once the battle commenced. The loss of the airfield allowed the Germans to gain a foothold on the island and the Allied forces eventually were evacuated from Crete. Despite his own performance during the battle, he received a bar to the Distinguished Service Order (DSO) that he had been awarded in the First World War. The fighting now shifting to North Africa, Hargest led his brigade during Operation Crusader in November 1941 but was captured by German forces. Held in a prisoner of war camp in Italy, he eventually escaped and was able to make his return to England in late 1943. He earned a second bar to his DSO for his efforts. He served as an observer with the British 50th Infantry Division for the Normandy landings in June 1944 and was killed by artillery just over two months later.