 On one occasion he had to command reliable troops in a nearby park as a unit of Guardsmen was briefly refusing to reembark for France, although the incident was resolved peacefully.The incident prompted an inquiry from the War Office as to whether the Guards Reserve Battalion "could be relied on".Macmillan then served in Ottawa, Ontario, Canada, in 1919 as ADC to Victor Cavendish, 9th Duke of Devonshire, then Governor General of Canada, and his future father-in-law.