On 29 March 1970, the North Vietnamese launched an offensive against the Cambodian army. Documents uncovered from the Soviet Union archives revealed that the invasion was launched at the explicit request of the Khmer Rouge following negotiations with Nuon Chea. A force of North Vietnamese quickly overran large parts of eastern Cambodia reaching to within of Phnom Penh before being pushed back. By June, three months after the removal of Sihanouk, they had swept government forces from the entire northeastern third of the country. 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.