Vietnam War: The Viet Cong commences combat with the Army of the Republic of Vietnam in the Battle of Đồng Xoài, one of the largest battles in the war.

The Vietnam War was an armed conflict in Vietnam, Laos, and Cambodia fought between North Vietnam and South Vietnam and their allies. North Vietnam was supported by the Soviet Union and China, while South Vietnam was supported by the United States and other anti-communist nations. The conflict was the second of the Indochina wars and a proxy war of the Cold War between the Soviet Union and US. The Vietnam War was one of the postcolonial wars of national liberation, a theater in the Cold War, and a civil war, with civil warfare a defining feature from the outset. Direct US military involvement escalated from 1965 until its withdrawal in 1973. The fighting spilled into the Laotian and Cambodian Civil Wars, which ended with all three countries becoming communist in 1975.

The Viet Cong (VC) was an epithet and umbrella term to refer to the communist-driven armed movement and united front organization in South Vietnam. It was formally organized as and led by the National Liberation Front of South Vietnam, and conducted military operations under the name of the Liberation Army of South Vietnam (LASV). The movement fought under the direction of North Vietnam against the South Vietnamese and United States governments during the Vietnam War. The organization had both guerrilla and regular army units, as well as a network of cadres who organized and mobilized peasants in the territory the VC controlled. During the war, communist fighters and some anti-war activists claimed that the VC was an insurgency indigenous to the South that represented the legitimate rights of people in South Vietnam, while the U.S. and South Vietnamese governments portrayed the group as a tool of North Vietnam. It was later conceded by the modern Vietnamese communist leadership that the movement was actually under the North Vietnamese political and military leadership, aiming to unify Vietnam under a single banner.

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 on 30 April 1975. Its predecessor was the ground forces of the Vietnamese National Army, established on 8 December 1950, representing Vietnam to fight in the First Indochina War against the communist Viet Minh rebels. 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.