Tonkin, Gulf of

Arm of the South China Sea, between the coast of Vietnam and the island of Hainan. There are oil reserves, and China and Vietnam disagree over their respective territorial boundaries in the area.

In August 1964 it was the scene of the Tonkin Gulf Incident between American destroyers and North Vietnamese vessels, which was used by President Johnson as the basis for a congressional resolution increasing US military involvement in the Vietnam War.