The Cambodia – Vietnam War (Khmer: សង្គ្រាមកម្ពុជា-វៀតណាម, Vietnamese: Chiến tranh Campuchia–Việt Nam), known in Vietnam as the Counter-offensive on the Southwestern border (Vietnamese: Chiến dịch Phản công Biên giới Tây-Nam), and by Cambodian nationalists as the Vietnamese invasion of Cambodia (Khmer: ការឈ្លានពានរបស់វៀតណាមមកកម្ពុជា), was an armed conflict between the former allies of Khmer Rouge state of Democratic Kampuchea and the Socialist Republic of Vietnam.

The war began after frequent border skirmishes and attacks from the Kampuchean Revolutionary Army on its border with Vietnam. It was also in response to numerous massacres of those of Vietnamese ancestry. The war occurred merely a few years after the two nations had been firms  for socialism and against American Imperialism.

On December 25th 1978 the Vietnamese launched a full scale invasion of Democratic Kampuchea. The Chinese advised the Khmer Rouge to seek peace negotiations, whilst the Khmer Rouge asked for Chinese military support, neither of which were to occur.

It took the Vietnamese forces, aided by many Khmer Rouge cadres worried by the the rule of Pol Pot just two weeks to overrun the Khmer Rouge.

On January 8th 1979 the capital of Phnom Penh had been taken and the beginnings of what would become the largely unrecognized Peoples Republic of Kampuchea and an ongoing civil war, with the China and USA supporting the now rebel Khmer Rouge.

The civil war would continue until 1998 in some way shape, or form. Vietnamese troops would remain in the country for s further 10 years.

In 1993 peace was largely restored (although not with the Khmer Rouge) and after a brief spell as the , the Kingdom of Cambodia with King Sihanouk as head of state was reintroduced.

The affects of the war between Vietnam and Cambodia are multi-faceted. Whilst they ended the tyrannical regime of Pol Pot, they also turned Cambodia into a Cold War proxy and led to the Khmer Rouge continuing to cause pain and suffering in the areas that they still controlled. It also turned Cambodia into one of the most land mined countries on earth, a legacy that still kills and maims people every year.

It also directly led to the punitive invasion of Vietnam by in February of 1979, in a short conflict that both sides were to claim victory in.