Answer: He wanted to take strong action against North Vietnam.
Historical context:
President Lyndon Johnson used alleged attacks by North Vietnamese against US ships in the Gulf of Tonkin as a reason to ramp up heavy war efforts against North Vietnam. The Gulf of Tonkin Resolution was passed by Congress in August, 1964, after reports of possible attacks on two US naval ships. The key wording in the resolution said:
<em>Resolved by the Senate and House of Representatives of the United States of America in Congress assembled, that the Congress approves and supports the determination of the President, as Commander in Chief, to take all necessary measures to repel any armed attack against the forces of the United States and to prevent further aggression.</em>
That resolution served as a blank check for President Johnson to send troops to whatever extent he deemed necessary in pursuance of the war. Johnson's strong actions taken against North Vietnam meant a huge increase of America's involvement in the war. Between 1964 and the end of Johnson's presidency in 1969, US troop levels in Vietnam increased from around 20,000 to over 500,000.
