The Khmer Krom (Khmer: khmaerkroom, Vietnamese: Kho Me Crom) are ethnically Khmer people living in the south western part of Vietnam, where they are recognized as one of Vietnam's fifty-three ethnic minorities. In the Khmer language, Krom means "lower" or "below", as it refers to an area of 89,000 square kilometres (34,363 sq mi) around modern day Saigon and the Mekong Delta, which used to be the southeasternmost territory of the Khmer Empire until its incorporation into Vietnam under the Nguyen lords in the early 18th century. This marks the final stage of the Vietnamese "March to the South" (nam tien).
Absorption of the Mekong Delta by Vietnam
In the 17th century a weakened Khmer state left the Mekong Delta poorly administered after repeated warfare with Siam. Concurrently Vietnamese refugees fleeing the Trinh-Nguyen War in Vietnam migrated into the area. In 1623 Cambodian king Chey Chettha II (1618-1628) officially sanctioned the Vietnamese immigrants to operate a custom house at Prey Nokor, then a small fishing village. The settlement steadily grew soon becoming a major regional port, attracting even more settlers.  In 1698 the Nguyen Lords of Hue commissioned Nguyen Huu Canh, a Vietnamese noble to organize the territory along Vietnamese administrative lines, thus by de facto detaching it from the Kingdom of Cambodia and incorporating it into Vietnam.  With the loss of the port of Prey Nokor, then renamed Sai Gon, 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 the Cambodian kingdom. By 1757 the Vietnamese had absorbed the provinces of Psar Dek (renamed Sa Dec in Vietnamese) on the Mekong itself, and Moat Chrouk (Vietnamized to Chau Doc) on the Bassac River.  Minh Mang enacted assimilation policies upon the Khmer such as forcing them to adopt Sino-Vietnamese surnames, culture, and clothing. Minh Mang sinicized ethnic minorities including the Cambodians, in line with Confucianism as he diffused Vietnamese culture with China's Han civilization using the term Han people Han Ren  for the Vietnamese. Minh Mang declared that "We must hope that their barbarian habits will be subconsciously dissipated, and that they will daily become more infected by Han [Sino-Vietnamese] customs." These policies were directed at the Khmer and hill tribes.  On June 4, 1949 the French President Vincent Auriol signed the accord reincorporating Cochinchina to Vietnam. This was done without consulting the indigenous Khmer-Krom. The legal transfer of control cut Cambodia off from the sea via the Mekong River as the Delta administered by an independent Vietnam. Left within the borders of Vietnam were the communities of Khmer people, nowadays Khmer Krom.

Q: What is the Mekong Delta?
A: In the 17th century a weakened Khmer state left the Mekong Delta poorly administered after repeated warfare with Siam.
What did Khmer do?
control of the area grew increasingly tenuous while increasing waves of Vietnamese settlers to the Delta isolated the Khmer of the Mekong Delta from the Cambodian kingdom.
control of the area grew increasingly tenuous while increasing waves of Vietnamese settlers to the Delta isolated the Khmer of the Mekong Delta from the Cambodian kingdom.