It is announced that the United States will send an additional 5,000 U.S. troops to Vietnam, bringing the total number of U.S. forces in Vietnam to 21,000. Military spokesmen and Washington officials insisted that this did not represent any change in policy, and that new troops would only intensify existing U.S. efforts. However, the situation changed in August 1964 when North Vietnamese torpedo boats attacked U.S. destroyers off the coast of North Vietnam. What became known as the Tonkin Gulf incident led to the passage of the Tonkin Gulf Resolution, which passed unanimously in the House and 88 to 2 in the Senate. The resolution gave the president approval to "take all necessary measures to repel an armed attack against the forces of the United States and to prevent further aggression." Using the resolution, Johnson ordered the bombing of North Vietnam in retaliation for the Tonkin Gulf incident.
In 1965, Johnson was faced with a rapidly deteriorating situation in Vietnam. The Viet Cong had increased the level of combat and there were indications that Hanoi was sending troops to fight in the south. It was apparent that the South Vietnamese were in danger of being overwhelmed. Johnson had sent Marines and paratroopers to protect American installations, but he had become convinced that more had to be done to stop the communists or they would soon overwhelm South Vietnam. While some advisers, such as Undersecretary of State George Ball, recommended a negotiated settlement, Secretary of Defense Robert McNamara urged the president to "expand promptly and substantially" the U.S. military presence in South Vietnam. Johnson, not wanting to "lose" Vietnam to the communists, ultimately accepted McNamara's recommendation. This decision led to a massive escalation of the war.