​​In May 1940, the British Expeditionary Force had fallen back from central Belgium and was encircled in the area around Dunkirk. An evacuation was being arranged under Royal Navy command—an operation known as Operation Dynamo.

​At that time, HMS ISIS was in dry dock and out of action, so Commander Clouston was assigned to a naval shore party under the command of Captain William Tennant. The party, consisting of eight officers and 160 men, was sent to Dunkirk to assist with the evacuation. On 27 May, they sailed aboard HMS Wolfhound.

​Three officers, including Clouston, decided to draw cards to determine where they would be deployed upon arrival. Clouston drew his card and was assigned to the eastern mole at Dunkirk. The mole stretched approximately one mile out to sea. It was a narrow structure—made of wood on a concrete base—wide enough for only four men to stand side by side. Its position made it an ideal, yet vulnerable, evacuation point and a clear target for the Luftwaffe. The initial goal of Operation Dynamo was to evacuate 45,000 men.

​Over the following days, Clouston’s efforts at the eastern mole were instrumental in establishing a controlled and efficient evacuation. He managed the movement of men onto the mole and coordinated the approach of vessels. His leadership under pressure helped to avoid chaos and speed up embarkation.

​Despite sustained Luftwaffe attacks, an estimated 60,000 men were evacuated from the mole alone. Reports suggest that two-thirds of all troops evacuated from Dunkirk did so under Clouston’s supervision. With a megaphone in hand, he issued clear and calm orders as up to 2,000 men per hour boarded rescue vessels. Word reached Dover quickly that the port was still operational and could continue to be used.

​Clouston’s efforts contributed directly to the successful evacuation of 224,686 British troops and 139,942 Allied troops during Operation Dynamo.