began as early as 1974, and the following year Pol Pot signed a treaty codifying the "friendship" between the Khmer Rouge and China. Diplomacy and military action 1975–1976: from fighting to building friendly relations The Fall of Phnom Penh and the Fall of Saigon in April 1975 immediately brought a new conflict between Vietnam and Cambodia. Although the North Vietnamese and the Khmer Rouge had previously fought side by side, the leaders of the newly created Democratic Kampuchea continued to view Vietnam with great suspicion, because they believed the Vietnamese communists had never given up their dream of creating an Indochinese federation with Vietnam as the leader. For that reason, the Kampuchean government removed all North Vietnamese military forces from Kampuchean territory shortly after their capture of Phnom Penh on 17 April 1975. In the first major clash between the two former allies, the Kampuchean Revolutionary Army (KRA) invaded the Vietnamese island of Phú Quốc on 1 May 1975 (barely 24 hours after Saigon fell), claiming it was historically part of Kampuchea's territory. Nine days later, on 10 May 1975, the KRA continued its incursion by capturing the Thổ Chu Islands, where it executed 500 Vietnamese civilians. The Vietnamese military immediately responded to Kampuchean actions by launching a counterattack and removing Kampuchean forces from Phú Quốc and Thổ Chu, and then invading the Kampuchean island of Koh Poulo Wai. In June 1975, while on a visit to Hanoi, Pol Pot proposed that Vietnam and his country should sign a treaty of friendship and begin discussions on border disputes. However, those discussions never materialised, and the Kampucheans claimed that Vietnam turned down both offers. In August 1975, Vietnam returned the island of Koh Poulo Wai to Kampuchea and formally recognised Kampuchean sovereignty over the island. Following those incidents, both countries attempted to improve their diplomatic relations with a series of congratulatory messages and exchange visits. On 17 April 1976, Vietnamese leaders sent a message to congratulate Khieu Samphan, Nuon Chea and Pol Pot on their "elections" as President, President of the People's Representatives and Premier of Kampuchea, respectively. Furthermore, the Vietnamese even denounced the alleged "U.S. bombing" of Siem Reap in February 1976, thereby reinforcing the Kampucheans' fictitious claims over the incident. In response, in June 1976, the Kampuchean leadership sent a message to the Provisional Revolutionary Government of the Republic of South Vietnam, which had governed South Vietnam since the