During the Vietnam War, Vietnamese and Cambodian communists had formed an alliance to fight U.S.-backed governments in their respective countries. Despite their cooperation with the Vietnamese, the Khmer Rouge leadership feared that the Vietnamese communists were planning to form an Indochinese federation, which would be dominated by Vietnam. In order to pre-empt any attempt by the Vietnamese to dominate them, the Khmer Rouge leadership began, as the Lon Nol government capitulated in 1975, to purge Vietnamese-trained personnel within their own ranks. Then, in May 1975, the newly formed Democratic Kampuchea began attacking Vietnam, beginning with an attack on the Vietnamese island of Phú Quốc.