The Third Indochina War was a series of interconnected armed conflicts, mainly among the various communist factions over strategic influence in Indochina after Communist victory in South Vietnam, Laos, and Cambodia in 1975. The conflict primarily started due to continued raids and incursions by the Khmer Rouge into Vietnamese territory that they sought to retake. These incursions would result in the Cambodian–Vietnamese War in which the newly unified Vietnam overthrew the Pol Pot regime and the Khmer Rouge, in turn ending the Cambodian genocide. Vietnam had installed a government led by many opponents of Pol Pot, most notably Hun Sen, a former Khmer Rouge commander. This led to Vietnam's occupation of Cambodia for over a decade. The Vietnamese push to completely destroy the Khmer Rouge led to them conducting border raids in Thailand against those who had provided sanctuary.
China strongly objected to the invasion of Cambodia. Chinese armed forces launched a punitive operation (Sino-Vietnamese War) in February 1979 and attacked Vietnam's northern provinces, determined to contain Soviet/Vietnamese influence and prevent territorial gains in the region.
In order to acquire full control over Cambodia the People's Army of Vietnam needed to dislodge the remaining Khmer Rouge leaders and units, which had retreated to the remote areas along the Thai-Cambodian border. After the Paris Peace Conference in 1989, the PAVN withdrew from Cambodian territory. Finally regular troop engagements in the region ended after the conclusion of the 1991 Paris Peace Accords.
In Laos, an insurgency continued until 2022, with the government being supported by both China and Vietnam.
Background
Soviet-Chinese discord
After Joseph Stalin's death in 1953, Nikita Khrushchev became leader of the Soviet Union. His denouncement of Stalin and his purges, the introduction of more moderate communist policies and foreign policy of peaceful coexistence with the West angered China's leadership. Mao Zedong had been following a strict Stalinistic course, that insisted on the cult of personality as a unifying force of the nation. Disagreements over technical assistance for developing China's nuclear weapons and basic economic policies further alienated the Soviets and the Chinese as opposing forces of communist influence across the globe. As decolonization movements began to pick up speed in the 1960s and many such countries descended into violence, both of the communist powers competed for political control of the various nations or competing factions in ongoing civil war fights. Ever more diverging Chinese and Soviet strategic and political doctrines had increased the Sino-Soviet split of the mid-1950s.
Political developments during the Vietnam War
The Democratic Republic of Vietnam (North Vietnam), which had chosen to ally with the USSR, justified incursions into neighbouring Laos and Cambodia during the Second Indochinese War by reference to the international nature of communist revolution, where "Indochina is a single strategic unit, a single battlefield" and the Vietnam People's Army's pivotal role in bringing this about. However, this internationalism was obstructed by complicated regional historical realities, such as the "timeless oppositions between the Chinese and the Vietnamese on the one hand and the Vietnamese and the Khmers on the other". North Vietnam intervened in the civil war between the Royal Lao Army and the communist Pathet Lao until the establishment of the Lao People's Democratic Republic and the "Treaty of Friendship and Cooperation" signed in July 1977. Permanently stationed North Vietnamese troops secured and maintained vital supply routes and strategic staging sites (Ho Chi Minh trail). From 1958 on, Northern and Southern Vietnamese combat troops also began to infiltrate the remote jungles of eastern Cambodia where they continued the Ho Chi Minh trail. The Cambodian communist insurgents had joined these sanctuaries during the late 1960s. Although co-operation took place, the Khmer communists did not adopt modern socialist doctrines and eventually allied with China.
The complete American withdrawal instantaneously eliminated the principal and common adversary of all the communist powers. The communist regimes of Cambodia, Vietnam and Laos pledged allegiance with one of these two opposing factions. The ensuing hostilities were fuelled by century-old animosities between Vietnam and Cambodia, and – particularly – Vietnam and China.