 On 29 March 1970 , the North Vietnamese had taken matters into their own hands and launched an offensive against the Cambodian army with documents uncovered from the Soviet archives revealing that the offensive was launched at the explicit request of the Khmer Rouge following negotiations with <unk> <unk> . A force of North Vietnamese quickly overran large parts of eastern Cambodia reaching to within 15 miles ( 24 km ) of Phnom Penh . After defeating those forces , the North Vietnamese turned the newly won territories over to the local insurgents . The Khmer Rouge also established " liberated " areas in the south and the southwestern parts of the country , where they operated independently of the North Vietnamese . 
