On 25 September 1978, the People’s Army of Vietnam () launched a full-scale invasion of Democratic Kampuchea. Although Vietnam had been engaged in sporadic armed conflicts with Democratic Kampuchea along land and maritime boundaries from as early as 1975, the division-sized invasion in 1978 was the first large-scale military operation that spelt the beginning of the Cambodian-Vietnamese War.
Although Vietnamese and Cambodian communists had aligned to fight the US and the Fall of Phnom Penh in April 1975, Pol Pot and the Khmer Rouge grew increasingly suspicious of a Vietnamese scheme to dominate . As a pre-emptive measure, the Khmer Rouge began purging Vietnamese-trained personnel within their own ranks as the Lon Nol regime capitulated. Later,
Despite a number of diplomatic exchanges, Khmer Rouge leaders continued to be wary of what they perceived as Vietnamese expansionism. On 30 April 1977, Democratic Kampuchea launched another major military attack in An Giang and Châu Đốc Provinces, killing hundreds of Vietnamese civilians. With the Khmer Rouge unwilling to engage in serious negotiations to settle the issue, Vietnam retaliated by attacking Svay Rieng Province in December 1977, which they took easily.
In the meantime, Sino-Cambodian relations had only grown stronger, while Vietnam’s relationship with China had deteriorated. With the political and military support the Khmer Rouge received from China during the Cambodian Civil War and the subsequent dominance of the Maoist Khmer Rouge regime over Democratic Kampuchea, Vietnam viewed their strengthening alliance as being pro-Chinese and too hostile towards Vietnam. This at a time when Vietnam increasingly allied itself with the Soviet Union’s ideologies of communism and socialism.
On 25 December 1978, 150,000 Vietnamese troops invaded Democratic Kampuchea. They overran the Kampuchean Revolutionary Army in just two weeks, thereby ending the excesses of Pol Pot’s regime, which had been responsible for the deaths of almost a quarter of all Cambodians between 1975 and December 1978. Vietnamese military intervention and the occupying forces’ subsequent allowing in of international food and aid to mitigate the massive famine is viewed as ending the Cambodian genocide.