the country receiving international aid. Laos and Cambodia By 1975, the North Vietnamese had lost influence over the Khmer Rouge. Phnom Penh, the capital of Cambodia, fell to the Khmer Rouge on 17 April 1975. Under the leadership of Pol Pot, the Khmer Rouge would eventually kill 1–3 million Cambodians out of a population of around 8 million, in one of the bloodiest genocides in history. The relationship between Vietnam and Democratic Kampuchea (Cambodia) escalated right after the end of the war. In response to the Khmer Rouge taking over Phu Quoc on 17 April and Tho Chu on 4 May 1975 and the belief that they were responsible for the disappearance of 500 Vietnamese natives on Tho Chu, Vietnam launched a counterattack to take back these islands. After several failed attempts to negotiate by both sides, Vietnam invaded Democratic Kampuchea in 1978 and ousted the Khmer Rouge, who were being supported by China, in the Cambodian–Vietnamese War. In response, China invaded Vietnam in 1979. The two countries fought a brief border war, known as the Sino-Vietnamese War. From 1978 to 1979, some 450,000 ethnic Chinese left Vietnam by boat as refugees or were expelled. The Pathet Lao overthrew the monarchy of Laos in December 1975, establishing the Lao People's Democratic Republic under the leadership of a member of the royal family, Souphanouvong. The change in regime was "quite peaceful, a sort of Asiatic 'velvet revolution'"—although 30,000 former officials were sent to reeducation camps, often enduring harsh conditions for several years. The conflict between Hmong rebels and the Pathet Lao continued in isolated pockets. Unexploded ordnance Unexploded ordnance, mostly from U.S. bombing, continues to detonate and kill people today and has rendered much land hazardous and impossible to cultivate. According to the Vietnamese government, ordnance has killed some 42,000 people since the war officially ended. In Laos, 80 million bombs failed to explode and remain scattered throughout the country. According to the government of Laos, unexploded ordnance has killed or injured over 20,000 Laotians since the end of the war and currently 50 people are killed or maimed every year. It is estimated that the explosives still remaining buried in the ground will not be removed entirely for the next few centuries. Refugee crisis Over 3 million people left Vietnam, Laos, and Cambodia in the Indochina refugee crisis after 1975. Most Asian countries were unwilling to accept these refugees, many