After the 1978 Vietnamese invasion of Cambodia and subsequent collapse of Democratic Kampuchea in 1979, the anti-Hanoi Khmer Rouge fled to the border regions of Thailand, and, with assistance from China, Pol Pot's troops managed to regroup and reorganize in forested and mountainous zones on the Thai-Cambodian border. During the 1980s and early 1990s Khmer Rouge forces operated from inside refugee camps in Thailand, in an attempt to de-stabilize the pro-Hanoi People's Republic of Kampuchea's government, which Thailand refused to recognise. Thailand and Vietnam faced off across the Thai-Cambodian border with frequent Vietnamese incursions and shellings into Thai territory throughout the 1980s in pursuit of Cambodian guerrillas who kept attacking Vietnamese occupation forces. Causes Thailand's suspicion of Vietnamese long-term objectives and fear of Vietnamese support for an internal Thai communist insurgency movement led the Thai government to support United States objectives in South Vietnam during the Vietnam War. In 1973 a new civilian government in Thailand created a chance for some degree of reconciliation with North Vietnam, when it proposed to remove United States military forces from Thai soil and adopt a more neutralist stance. Hanoi responded by sending a delegation to Bangkok, but talks broke down before any progress in improving relations could be made. Discussions resumed in August 1976, after Hanoi had defeated the South Vietnamese and united the country under its rule. They resulted in a call for an exchange of ambassadors and for an opening of negotiations on trade and economic co-operation, but a military coup in October 1976 ushered in a new Thai government less sympathetic to the Vietnamese communists. Contact was resumed briefly in May 1977, when Vietnam, Thailand, and Laos held a conference to discuss resuming work on the Mekong Development Project, a major cooperative effort that had been halted by the Vietnam War. Beginning in December 1978, however, the conflict in Cambodia dominated diplomatic exchanges, and seasonal Vietnamese military offensives that included incursions across the Thai border and numerous Thai casualties particularly strained the relationship. In 1979, after Vietnam's military occupation of Cambodia Bangkok allied itself with the Khmer Rouge, an adversary of Vietnam and looked to Beijing for security assistance. In both instances, Thailand's actions hardened Hanoi's attitude toward Bangkok. As the ASEAN member most vulnerable to a hypothetical Vietnamese attack for having given shelter to the Khmer Rouge in camps within its territory, Thailand was foremost among the ASEAN partners