1. Jean Decoux was promoted to aspirant second class in 1903, to aspirant first class the following year, ship-of-the-line ensign in 1906, ship-of-the-line lieutenant in 1913, corvette captain in 1920, frigate captain in 1923, ship-of-the-line captain in 1929 and rear admiral in 1935.
Jean Decoux was appointed commander of the defence sector at Toulon in 1938 and promoted to vice-admiral.
On 13 January 1939, Jean Decoux was appointed Commander-in-Chief of the Naval Forces in the Far East by President Albert Lebrun.
Jean Decoux assumed his new appointment, with the rank of squadron vice-admiral, on 12 May When the French Navy submarine Phenix sank with the loss of all hands in the South China Sea during a training exercise on 15 June 1939, he ordered the light cruiser Lamotte-Picquet to put to sea to join the submarine L'Espoir in searching for her.
Jean Decoux cabled his Vichy superiors for aid, but when no help was forthcoming, he signed a treaty on 20 September 1940 that opened Haiphong Harbour to the Japanese and gave them the right to station troops in the region.
Jean Decoux worked to improve relations between French colonists and the Vietnamese.
Jean Decoux established a grand federal council containing twice as many Vietnamese members as Frenchmen and installed Vietnamese in civil-service positions with equal pay to that of French officials.
The Indochinese Federal Council, which was composed only of Indochinese, and later the Grand Federal Council were the formal structures that Jean Decoux felt he needed to build to develop the Indochinese federal consciousness simultaneously with the elevation of the elite.
Jean Decoux believed that would reverse the reluctance of the local population to accept the politics of collaboration.
Jean Decoux is buried in the Domaine de Marie Church convent in Da Lat.
Jean Decoux enforced the discriminatory Vichy laws against Gaullists and Freemasons, as well as the Vichy anti-Jewish legislation, but he decried the impact on the French colonial regime and society.
One writer claims that Jean Decoux remained unconcerned by the famine of 1945.
Over one million Vietnamese died of starvation in the countryside and urban cities and the author asserts that the Jean Decoux government did nothing to help the Vietnamese peasants, farmers, and poor, despite soliciting and courting the Vietnamese elite.
On 9 March 1945, the Japanese took direct control of the government and ousted Jean Decoux, establishing the Empire of Vietnam.
Arrested, Jean Decoux was tried by Justice High Court and placed under house arrest.
Jean Decoux later wrote the book A la barre de l'Indochine.
Jean Decoux died in Paris in 1963 and is buried in Annecy.