Short answers, pulled from the story.
The Cambodian-Vietnamese War began on the 25th of December 1978, when Vietnam launched a full-scale invasion of Cambodia with an estimated 150,000 troops. It ended with the withdrawal of the last 26,000 Vietnamese soldiers between the 21st and the 26th of September 1989, concluding a ten-year occupation.
Vietnam invaded Cambodia in December 1978 to remove the Khmer Rouge government, which it identified as a Chinese proxy hostile to Vietnamese interests. Years of Khmer Rouge border attacks, including the Ba Chuc massacre of over 3,000 Vietnamese civilians in April 1978, combined with Vietnam's conclusion that conventional military force was the only effective means of regime change.
The Khmer Rouge killed between 1.2 and 2.8 million people between 1975 and December 1978, representing between 13 and 30 percent of Cambodia's entire population. The Vietnamese invasion, and the food aid the occupying forces subsequently facilitated, ended the genocide.
China was the primary supporter of the Khmer Rouge, providing weapons, military hardware, and between 10,000 and 20,000 military and civilian advisers. The United States, ASEAN members, and most UN member countries also backed the Khmer Rouge's diplomatic recognition at the UN, even as they opposed its human rights record.
The Coalition Government of Democratic Kampuchea was a resistance alliance formed on the 22nd of June 1982 by three factions opposed to the Vietnamese-installed government: the Khmer Rouge, Norodom Sihanouk's FUNCINPEC, and Son Sann's Khmer People's National Liberation Front. Sihanouk served as president, Khieu Samphan as vice-president in charge of foreign affairs, and Son Sann as prime minister.
Vietnam suffered 15,000 soldiers killed and 30,000 wounded during the ten-year occupation. The occupation also cost Vietnam severely in economic and diplomatic terms: the United States blocked Vietnam from the World Bank, the Asian Development Bank, and the International Monetary Fund; Japan suspended all economic aid in 1979; and nearly 80 countries continued to recognise the deposed Khmer Rouge government as late as January 1980.