The Mekong Delta was once part of the Khmer kingdom, and was the last
region of modern-day Vietnam to be annexed and settled by the
Vietnamese. Cambodians, mindful that they controlled the area until the
18th century, still call the delta 'Lower Cambodia'.
The Khmer Rouge tried to follow up on this claim by raiding Vietnamese villages and massacring the inhabitants. This led the Vietnamese army to invade Cambodia in 1979 and oust the Khmer Rouge from power. Most of the current inhabitants of the Mekong Delta are ethnic Vietnamese, but there are also significant populations of ethnic Chinese and Khmer, as well as a few Chams.
When the government introduced collective farming to the delta in 1975, production fell significantly and there were food short-ages in Saigon (although farmers in the delta easily grew enough to feed themselves). People from Saigon would head down to the delta to buy sacks of black-market rice, but to prevent 'profiteering', the police set up checkpoints and confiscated rice from anyone carrying more than 10kg. All this ended in 1986 and farmers in this region have since propelled Vietnam forward to become the world's second-largest rice exporter after Thailand.