In December 1941, the Pacific War broke out and the Japanese Army started an unexpected invasion to Hong Kong. During the Battle of Hong Kong, Holmes belonged to the British Forces Overseas Hong Kong but was soon summoned to serve under the Special Operations Executive (SOE). His major duty was to attack the Japanese army from the rear area. However, when then Governor Sir Mark Young surrendered on 25 December, Holmes successfully managed to flee to the mainland China. Thus he was not caught by the Japanese Army and was not a prisoner of war throughout the Second World War. His only brother, Captain Leslie Benjamin Holmes (1905–1941) of the Hong Kong Volunteer Defence Corps, was killed in action in Hong Kong on 19 December. He died at the age of 36 and left his widow, Marguerite Julia Holmes, of Ramsbury, Wiltshire. Today, his grave can still be found in the Stanley Military Cemetery.