(1977-1991)-- During their war against the U.S.-sponsored
regimes in Saigon and Phnom Penh, the North Vietnamese and the
Khmer Rouge were able to mask their ideological differences and
ignore the historical hostility between their two peoples. After
taking power though, these differences turned violent. Beginning
with low-level cross-border raids and escalating into full-fledged
war in late December of 1978 when Vietnam launched a massive
conventional invasion of Cambodia, swiftly occupying the nation
Vietnam set up a new
government in Phnom Penh with Khmer Rouge defectors but found
itself immersed in a long and difficult war of occupation as the
Khmer Rouge returned to the guerrilla warfare they knew so well.
Vietnamese troops left after more than a decade, with the friendly
government of Heng Samrin in control of most of Cambodia.