The Cambodian–Vietnamese War (, ), known in Vietnam as the Counter-offensive on the Southwestern border (), and by Cambodian nationalists as the Vietnamese invasion of Cambodia (), was an armed conflict between Democratic Kampuchea, controlled by the Khmer Rouge, and the Socialist Republic of Vietnam. The war began with repeated attacks by the Liberation Army of Kampuchea on the southwestern border of Vietnam, particularly the Ba Chuc massacre which resulted in the deaths of over 3,000 Vietnamese civilians. On 25 December 1978, Vietnam launched a full-scale invasion of Kampuchea, and subsequently occupied the country and removed the government of the Communist Party of Kampuchea from power. During the Vietnam War, Vietnamese and Cambodian communists had formed an alliance to fight U.S.-backed governments in their respective countries. Despite their cooperation with the Vietnamese, the Khmer Rouge leadership feared that the Vietnamese communists were planning to form an Indochinese federation, which would be dominated by Vietnam. They were also harboring irredentist sentiments about the Mekong Delta region. In order to pre-empt any attempt by the Vietnamese to dominate them, the Khmer Rouge leadership began, as the Lon Nol government capitulated in 1975, to purge Vietnamese-trained personnel within their own ranks. Then, in May 1975, the newly formed Democratic Kampuchea began attacking Vietnam, beginning with an attack on the Vietnamese island of Phú Quốc. In spite of the fighting, the leaders of reunified Vietnam and Kampuchea made several public diplomatic exchanges throughout 1976 to highlight the supposedly strong relations between them. However, behind the scenes, Kampuchean leaders continued to fear what they perceived as Vietnamese expansionism. Therefore, on 30 April 1977, they launched another major military attack on Vietnam. Shocked by the Kampuchean assault, Vietnam launched a retaliatory strike at the end of 1977 in an attempt to force the Kampuchean government to negotiate. The Vietnamese military withdrew in January 1978, even though its political objectives had not been achieved; the Khmer Rouge remained unwilling to negotiate seriously. Small-scale fighting continued between the two countries throughout 1978, as China tried to mediate peace talks between the two sides. However, the two governments could not reach a compromise. By the end of 1978, Vietnamese leaders decided to remove the Khmer Rouge-dominated government of Democratic Kampuchea, perceiving it as being pro-Chinese and hostile towards Vietnam. On 25 December 1978, 150,000 Vietnamese troops invaded Democratic Kampuchea and overran the Kampuchean Revolutionary Army in just two weeks,