Following World War II, Thơ became Interior Minister in the French-backed State of Vietnam under former Emperor Bảo Đại. Following the withdrawal of France from Indochina after the Battle of Điện Biên Phủ, Vietnam was partitioned into a communist north and anti-communist south. Following the proclamation of the Republic of Vietnam—commonly known as South Vietnam—by President Ngô Đình Diệm, who had dethroned Bảo Đại in a fraudulent referendum, Thơ was appointed the inaugural ambassador to Japan. Despite spending most of his time in Tokyo confined to his bed by a fractured hip, Thơ secured reparations from Japan for its imperial occupation of Vietnam during World War II.