Answer:
Cambodia, Vietnam and Laos all share a major river that empties into the South China Sea, and that was also the site of many battles during the Vietnam War. It is the Mekong River.
Step-by-step explanation:
The Mekong River is a long Southeast Asian river flowing in a slightly southerly direction through six countries - China, Burma, Laos, Thailand, Cambodia and Vietnam - until it drains into the South China Sea. It has a length of 4880 km, which makes it the eighth longest river in the world.