Nguyễn Khánh
Col. Viên was a critical supporter of the January 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. By March 14, Viên had been promoted by the new regime to brigadier general.