Khmer King influenced by the Siam. Siam under controlled of British army while the Vietnam colonized by the France. Cambodia was under the protectorate of France only. Cambodia asked to moved Koh Tral to the France protection.
Later, Koh Tral belonged to the Co-Chinchin (South Vietnam, North Vietnam and Kampuchea Krom. Years then, The North Vietnam controlled the South Vietnam till today. Due to the Koh Tral belong to Kampuchea Krom, the part of South Vietnam thus this biggest island belong to Vietnam!