At the outbreak of war in July 1914 he was the second captain of a steamship, but he soon decided to leave the merchant navy to join the infantry chasseurs to defend his country.

Named sous lieutenant in December, he could not bear the static life of the trenches and moved to the nascent French Air Force, first as an observer-corps officer, then (briefly in July 1915) as a pilot.

After carrying out many observation and bombing missions, he was injured in battle, mentioned in despatches by the French and Italian navies and knighted by the king of Italy.

Promoted lieutenant de vaisseau in the reserve, he was then put on attachment (because of his long training as captain) as liaison officer to the under secretary of state to the merchant navy until his demobilization in April 1919.

On the president's suggestion, Ducuing, the French Navy in June 1927 created training centres for reserve officers, that will become accessible to naval NCOs from 1931.

At the end of January 1940, the French Admiralty was anxious to observe and maintain free passage along the Pas-de-Calais coast, and so set up a coastal battery at Cap Gris-Nez.

When practically encircled, commandant Ducuing and his men completely destroyed a German column made up of a small number of side-cars, a light tank and two armored cars.