Cao Văn Viên was a four-star army general in the Army of the Republic of Vietnam during the Vietnam War. He rose to the position of Chairman of the South Vietnamese Joint General Staff. Considered one of "the most gifted" of South Vietnam's military leaders, he was previously called an "absolute key figure" and one of "the most important Vietnamese military leaders" in the U.S.-led fighting during the Vietnam War. Along with Trần Thiện Khiêm he was one of only two four-star generals in the entire history of South Vietnam.

The Army of the Republic of Vietnam composed the ground forces of the South Vietnamese military from its inception in 1955 to the Fall of Saigon in April 1975. At the ARVN's peak, an estimated 1 in 9 citizens of South Vietnam were enlisted, composed of Regular Forces and the more voluntary Regional Forces and the Popular Force militias. It is estimated to have suffered 1,394,000 casualties during the Vietnam War.