Born in Laos, Cao Van Vien became a ranking officer in the army of the Associated State of Vietnam. He graduated from the Lycée Pavie in Vientiane in 1940 as the Japanese entered Indochina. Little is known about his whereabouts during World War II. During the Indochina conflict, he served in the French Union forces as a non-commissioned interpreter-translator (French-Vietnamese). In 1949, the French selected him as one of the first NCOs for training as an officer in the emerging Vietnamese national army. He entered the Centre de perfectionnement de sous-officiers in Vung Tau. Following graduation, he worked in the army’s recruiting office and, in 1950, moved on to work in the press office of the department of Defense in the Associated State of Vietnam. In 1952, Cao Van Vien continued to rise in the army, studying at the Hanoi Command and Staff School with the likes of Nguyen Van Thieu. He then assumed command of the 10th Battailon and led troops in battles against the Democratic Republic of Vietnam. In early 1954, Cao Van Vien served as assistant chief of staff for Intelligence and for Logistics in the 3rd Military Zone before becoming commanding officer of the 56th infantry battalion at the end of the Indochina War.