According to Republic of Vietnam, Poulo Wai was a part of Vietnam since the 18th century and the island was under Cambodian administrative management in 1939 in accordance with the decisions of French colonists. Vietnam has recognized Poulo Wai as part of Cambodia since 1976, and the recognition is seen as a sign of goodwill by Vietnam to preserve its relationship with Cambodia. The following month, Pol Pot, Nuon Chea, and Ieng Sary travelled secretly to Hanoi in May, where they proposed a Friendship Treaty between the two countries. In the short term, this successfully eased tensions. Although the Vietnamese evacuated Poulo Wai in August, incidents continued along Cambodia's northeastern border. At the instigation of the Phnom Penh regime, thousands of Vietnamese also were driven out of Cambodia. In May Cambodian and Vietnamese representatives met in Phnom Penh in order to establish a commission to resolve border disagreements. The Vietnamese refused to recognize the Brévié Line—the colonial-era demarcation of maritime borders between the two countries—and the negotiations broke down. In late September, however, a few days before Pol Pot was forced to resign as prime minister, air links were established between Phnom Penh and Hanoi. With Pol Pot back at the forefront of the regime in 1977, the situation rapidly deteriorated. Incidents escalated along all of Cambodia's borders. Khmer Rouge forces attacked villages in the border areas of Thailand near Aranyaprathet. Brutal murders of Thai villagers, including women and children, were the first widely reported concrete evidence of Khmer Rouge atrocities. There were also incidents along the Laos border. At approximately the same time, villages in Vietnam's border areas underwent renewed attacks. In turn, Vietnam launched air strikes against Cambodia. From 18 to 30 April 1978, Cambodian troops, after invading the Vietnamese province of An Giang, carried out the Ba Chúc massacre causing 3,157 civilian deaths in the province of Tây Ninh, Vietnam. In September, border fighting resulted in as many as 1,000 Vietnamese civilian casualties. The following month, the Vietnamese counter-attacked in a campaign involving a force of 20,000 personnel. Vietnamese defense minister General Võ Nguyên Giáp underestimated the tenacity of the Khmer Rouge, however, and was obliged to commit an additional 58,000 reinforcements in December. On 6 January 1978, Giap's forces began an orderly withdrawal from Cambodian territory. The Vietnamese apparently believed they had "taught a lesson" to the Cambodians, but Pol Pot proclaimed this a "victory" even