The Third Indochina War was a series of interconnected military conflicts among the various communist factions over strategic influence in mainland Southeast Asia 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.
Vietnam-China relations became tense because Vietnam chose to be pro-Soviet after unification in 1976 instead of being neutral as before. 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.