In addition to their continuing internal challenges, Southeast Asian states have been hampered by serious tensions among themselves. Some of these tensions were a consequence of historical rivalries and territorial disputes that had been submerged during the long era of colonial rule. Cambodia, for example, has bickered with both of its neighbors, Thailand and Vietnam, over mutual frontiers drawn up originally by the French for their own convenience. After the reunification of Vietnam under Communist rule in 1975, the lingering border dispute between Cambodia and Vietnam erupted again. In April 1975, a brutal revolutionary regime under the leadership of the Khmer Rouge dictator Pol Pot came to power in Cambodia and proceeded to carry out the massacre of more than one million Cambodians. Then, claiming that vast territories in the Mekong delta had been seized from Cambodia by the Vietnamese in previous centuries, the Khmer Rouge regime launched attacks across the common border. In response, Vietnamese forces invaded Cambodia in December 1978 and installed a pro-Hanoi regime in Phnom Penh. Fearful of Vietnamís increasing power in the region, China launched a brief attack on Vietnam to demonstrate its displeasure. The outbreak of war among the erstwhile Communist allies aroused the concern of other countries in the neighborhood. In 1967, several non-Communist countries had established the Association of Southeast Asian Nations (ASEAN). Composed of Indonesia, Malaysia, Thailand, Singapore, and the Philippines, ASEAN at first concentrated on cooperative social and economic endeavors, but after the end of the Vietnam War, it cooperated with other states in an effort to force the Vietnamese to withdraw. In 1991, the Vietnamese finally withdrew, and a new government was formed in Phnom Penh. The growth of ASEAN from a weak collection of diverse states into a stronger organization whose members cooperate militarily and politically has helped provide the nations of Southeast Asia with a more cohesive voice to represent their interests on the world stage. They will need it, for disagreements with Western countries over global economic issues and the rising power of China will present major challenges in coming years. That Vietnam was admitted into ASEAN in 1996 should provide both Hanoi and its neighbors with greater leverage in dealing with their powerful neighbor to the north.