Col. Viên was a critical supporter of the 1964 South Vietnamese coup in which President Dương Văn Minh was toppled by General Nguyễn Khánh, plotting with him to overthrow Minh and successfully ordering his Airborne Division troops to help secure the capital [REF].
Viên was appointed Chief of Staff of the Joint General Staff (JGS) on September 11, 1964, after President Khanh dismissed General Nguyễn Văn Thiệu in order to win Buddhist support for his government [REF].