The shift toward a more aggressive Cambodian policy came in early 1977 when a directive from “870,” the party’s standing committee, enjoined regional cadre to round up ethnic Vietnamese and turn them over to state security. There were few Vietnamese in Cambodia in 1977. Most of them had been repatriated under Lon Nol or after April 1975. Those who remained, often Vietnamese women married to Khmer, were hunted down and killed. In March, military forces in the east were taken off agricultural duties and moved en masse to frontier regions, foreshadowing an assault. Soon after, in the southwest, several cross-border attacks occurred, timed to coincide, it seems, with the second anniversary of the liberation of Saigon. The Vietnamese towns of Chaudoc and Hatien were shelled by Khmer artillery, and small-unit attacks, initiated by the Khmer, took place along the border, inflicting heavy casualties on civilians and an unprepared militia. In a tragic irony, many of those killed were ethnic Khmers whom Democratic Kampuchea in its propaganda proposed to liberate from the Vietnamese. For the time being, none of the attacks was publicized by either side. A Vietnamese proposal for a cease-fire in June 1977 was turned down, and units in the eastern zone were secretly advised to prepare for large-unit offensive actions.40

Pol Pot’s anxiety about Vietnam increased in July 1977 when Vietnam signed a treaty of friendship with Laos. By then, documents from S-21 had confirmed Pol Pot’s suspicions that the Cambodian Communist party was riddled with Vietnamese agents who had undermined Cambodia’s economic revolution and were endangering the leaders of the party. The documents focused on “enemies” in the eastern zone, where fighting against the Vietnamese was being pursued with the greatest vigor. In August and September, Cambodian artillery units bombarded Vietnamese territory, and there were several incursions near Tay Ninh. The most significant of these occurred when elements of a Red Khmer brigade deeply penetrated the province, massacred hundreds of civilians, and hauled prisoners and livestock back across the border. The attack may have been an attempt by eastern zone cadre, under Sao Phim’s leadership, to prove their loyalty to the party center. It also demonstrated commendable aggressiveness, which Pol Pot would point out in his upcoming discussions in Beijing. Vietnamese probes allegedly took place at the same time, “all along the border.”41