In June 1978, the VPAF started bombing KRA positions along the border regions, flying about 30 bombing sorties per day and inflicting heavy casualties on the Kampucheans. By that stage in the conflict, most surviving leaders of the Eastern Military Zone had escaped into Vietnam, where they assembled at various secret camps with the purpose of forming a Vietnamese-backed "liberation army" to fight against the Khmer Rouge government. Meanwhile, the Vietnamese Communist Party Politburo was meeting in Hanoi to discuss its strategy for Kampuchea. It concluded that the Khmer Rouge government was a proxy of China, which had been trying to fill the power vacuum following the withdrawal of the United States. As such, China was identified as Vietnam's main enemy, and its client government in Phnom Penh had to be removed by conventional military force, because the Vietnamese adaptation of the Maoist "people's war" doctrine had not been a success against the Khmer Rouge's security apparatus.