In December 1976, the Kampuchean Communist Party Central Committee's annual plenum proposed the country ready itself for the prospect of war with Vietnam. Pol Pot believed that Vietnam was committed to expansionism and thus was a threat to Cambodian independence. There were renewed border clashes between Cambodia and Vietnam in early 1977, continuing into April. On 30 April, Cambodian units, backed by artillery fire, entered Vietnam and attacked a series of villages, killing several hundred Vietnamese civilians. Vietnam responded by ordering its Air Force to bomb Cambodian border positions. Several months later, the fighting resumed; in September, two divisions of the Cambodian Eastern Zone entered the Tay Ninh area of Vietnam, where they attacked several villages and slaughtered their inhabitants. That month, Pol Pot travelled to Beijing, and from there to North Korea, where Kim Il Sung spoke out against Vietnam in solidarity with the Khmer Rouge.