Like his predecessor, Decoux initially wished to continue the fight against the Axis powers, but he swore allegiance to Pétain's regime after realizing that his meager armed forces were no match for the Japanese.

Decoux reportedly received demands from the Japanese in early August for permission to move troops through Tonkin (later Vietnam) in order to build air bases and block Allied supply routes to China.

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.

They gave their opinions to the Governor General to assist him in his decision-making and served as a forum to strengthen relations between these elites and the authorities.

In addition, he apparently wished to show goodwill toward the Indochinese peoples after Roosevelt's public statement that maintaining French sovereignty in Indochina was not a principal objective of the United States.

[6] However, archival records show that Allied bombardment of railways and the requisitioning of boats by the Japanese made it impossible to transport rice from Cochinchina to Tonkin.

Cared for in the Val de Grâce Military Hospital in Paris, he was declared innocent of all charges in 1949 and restored to his former rank and privileges.