Final answer:
In 1964, in response to the Gulf of Tonkin incidents, President Johnson sent U.S. troops to Vietnam under the authority of the Gulf of Tonkin Resolution.
Step-by-step explanation:
In 1964, in response to the Gulf of Tonkin incidents, during which two U.S. war vessels were allegedly attacked by North Vietnamese naval vessels, President Johnson sent troops to Vietnam. This marked the start of the U.S.'s major military involvement in the Vietnam War. It's important to note that this action was authorized by the Gulf of Tonkin Resolution passed by the U.S. Congress.
Learn more about Gulf of Tonkin Resolution