When LBJ became President there were some 16,000 advisers in South Vietnam and some of those were involved in combat. The President's main concern at the time was the war on poverty and the building of what he called the Great Society. The strategic hamlets that were built by the South Vietnamese government were being destroyed, often with the help of those lived there. Hanoi decided to escalate the war and Johnson found himself in an election against a conservative candidate. He was under pressure not to relent in the fight against Communism. On August 4, 1965, the USS Maddox was attacked in the Gulf of Tonkin (though some now question the veracity of the reports around that incident). The US Congress passed the Gulf of Tonkin resolution that essentially authorized the President to make war. Johnson ordered the bombing of the North, in an operation called Rolling Thunder. A series of attacks in Saigon - the explosion at the Brinks hotel; four days later, a major attack on the South Vietnamese army; and then an attack on Pleiku - led to the first request for additional US troops to protect the three jet-capable US airfields. On March 8, 1965 3500 Marines landed at Da Nang and by the end of the year, there would be 2000,000 American troops in Vietnam.
—garykmcd