Douglas was sent to Rear Admiral Lord Louis Mountbatten's Combined Operations Headquarters in London in the summer of 1942. Afterwards he was ordered by Mountbatten to duties as Commander of Flotilla of Amphibious Raiding Craft in raids across the English Channel. He was the only American officer assigned there. In 1945, he was promoted to full Commander. In 1952 he was became a Captain in the Naval Reserve.