[mr] In the first half of the first millennium, the polity of Funan dominated the Mekong delta region. Our Chinese historical sources record a flourishing maritime trade which is confirmed by archeological evidence, including trade goods from all over Asia and coins from as far away as the Roman Empire. The trade goods, a complicated canal system, fertile lands, and early temple complexes suggest a dense population and a hierarchical society. The power of Funan declined as Chenla and later Angkor, polities further upriver in modern-day Cambodia, became more influential and less trade-oriented.
After Angkor toppled and the Khmer royalty moved the capital to Phnom Penh, long-distance trade increased up the Mekong delta routes, although the delta region was now sparsely populated. Political control over the region was weak until the Vietnamese took over the region in the late 18th century. The Vietnamese state encouraged the settlement of the delta area as a frontier, and it became something of a wild west of loose political control, diverse peoples, and criminals. Intensifying around 1800, the French took advantage of factional strife and technological advantages, and gradually claimed and took over all of Vietnam, beginning with Saigon and South Vietnam.
The Mekong delta is a famous birthplace of religious sects. It was also the site of the first nationalist revolt against French colonialism, and a battleground between Viet Cong and US/SVN forces during the Vietnam War. Since Vietnam's independence and reunification, the region provides much of Vietnam’s rice, fish, and shrimp surpluses for export, although much of its population remains poor.
From my course notes on lectures by Profs. Liam Kelley and Michael Aung-Thwin, as well as Wikipedia: Funan, History of Vietnam. [mr]