History A weakened Khmer

A weakened Khmer state after repeated warfare with Siam in the 17th century left the Mekong Delta poorly administered. Concurrently Vietnamese refugees fleeing the Trịnh–Nguyễn War in Vietnam pushed into the area. Cambodian king Chey Chettha II (1618–1628) in 1623 officially sanctioned the Vietnamese to operate a custom house at Prey Nokor, then a small fishing village. The settlement grew steadily as a major regional port, attracting even more settlers.
The Nguyễn Lords of Huế in 1698 commissioned Nguyễn Hữu Cảnh, a Vietnamese noble, to organize the territory along Vietnamese administrative lines, thus by de facto detaching it from Cambodia and joining it to Vietnam.
With the loss of the port of Prey Nokor, then renamed Sài Gòn, Cambodia's control of the area grew increasingly tenuous while increasing waves of Vietnamese settlers to the Delta isolated the Khmer of the Mekong Delta from their brethren in Cambodia proper. By 1757, the Vietnamese had absorbed the provinces of Psar Dèk (renamed Sa Đéc in Vietnamese) on the Mekong itself, and Moat Chrouk (Vietnamized to Châu Đốc) on the Bassac River.
On June 4th 1949, the French President, Vincent Auriol, signed the law granting Cochin-China to the Bao Dai government without consultation of the indigenous Khmer-Krom. Cambodia was then cut off from direct access to the South China Sea at that point. Left within the borders of Vietnam were large pockets of Khmer people, now known as the Khmer Krom.