American combat troops would be sent to prevent the collapse of the ARVN and, if need be, to root out the Viet Cong, but this would take time. In the meantime, a carefully orchestrated air offensive against North Vietnam, ROLLING THUNDER, conducted in part by Air Force fighter bombers sent to Thailand in the wake of the Gulf of Tonkin incidents and held in reserve, would convince the communists that the United States meant business, hopefully leading to prompt negotiations to end the war in the south. ROLLING THUNDER, formally initiated on 5 March, was intended to satisfy no less than five strategic objectives: Secretary of Defense McNamara saw the bombing offensive primarily as an instrument of graduated escalation, an application offeree to achieve a negotiated settlement with minimum expenditure ofblood and money. General Westmoreland, less sanguine about the prospects for negotiations and responsible for preventing a Viet Cong takeover in the south, saw it as a justification for introducing more U.S. ground troops into South Vietnam; brought in to defend U.S. air bases from mortar and sapper attacks, they could then be used for search and destroy missions while the ARVN concentrated on pacification. National Security Policy Advisor McGeorge Bundy saw the air attacks on North Vietnam as a means of bolstering South Vietnamese resolve, convincing the hard-pressed ARVN that they would not be abandoned. Senior military commanders outside Vietnam, specifically CINCPAC (Commander in Chief Pacific), Admiral U. S. Grant Sharp, and the joint chiefs of staff envisioned ROLLING THUNDER as an air interdiction campaign against communist lines of resupply: an effort to choke off supplies to Viet Cong and PAVN main force units in the south and bring about the collapse of the North Vietnamese transportation system. The tensions between these disparate objectives were never resolved, with consequences for the air war against North Vietnam which are addressed in Chapter 6. More to the point, the Third Marines began landing near Da Nang on 8 March and were shortly providing air-base security. By the end of the month, they were joined by the 1 st Marine Air Wing from Okinawa and Japan and by 9th Marine Brigade headquarters. Initially, U.S. authorities observed the fiction that the Marines were present only to provide air-base security, but operational logic quickly won out: to provide security, an infantry force must patrol beyond the perimeter to obtain intelligence and to forestall attack, and once outside the perimeter, it must act aggressively to survive. On 6 April, 1965, President Johnson authorized U.S. ground forces in Vietnam to engage in offensive operations. Whatever doubt might have existed up to this point was dispelled; the United States was at war.
With the advantage of dedicated Navy amphibious transports and logistical support vessels, the Marines were first to deploy. But the Army was not far behind. The173rd Airborne Brigade, airlifted into Vung Tau and Bien Hoa in May, was the first Army ground combat unit to arrive. It was followed in June by the first contingent provided by an American ally, the 1 st Battalion, Royal Australian Regiment. General Westmoreland lost no time putting the new forces to use, mounting a major search and destroy operation - the first of the war - in War Zone D north of Saigon. The arrival of ground combat troops was accompanied by an increase in air strength as Air Force fighter bomber and transport squadrons occupied hurriedly constructed bases. In June President Johnson authorized strikes by B-52 bombers of the Strategic Air Command (SAC) based onGuam under the code name ARC LIGHT. Converted from their role as part of SAC'S nuclear deterrent force by the provision of conventional bomb shackles and external racks, the B-52s were to play a major role in the war. Arriving silently - they flew at altitudes too high to be seen or heard from the ground under normal circumstances - the B-52s introduced an element of terror into the lives of communist cadres in base areas deep in the jungle. Capable of pinpoint accuracy, the B-52s proved effective in direct support of U.S. and ARVN forces and were to prove a fearsome equalizer in battles to come.
By July, the 1st Infantry and 101 st Airborne Divisions had begun to arrive In Vietnam along with a New Zealand artillery battery, and more Marine and Army units were not far behind. The elite 1 st Cavalry Division (Airmobile), an experimental organization equipped with its own helicopters for full air mobility, arrived in September and in October another ally weighed in with the arrival of the Republic of Korea (ROK) Capital Division and Marine Brigade. In December, the 25th Division began deploying to Vietnam from Hawaii; others were to follow until, in February of 1967, the Army ran out of deployable divisions and formed the America) Division from smaller units already in Vietnam. By the end of 1965, much of the force structure which was to fight the American war was in place. The line infantry, artillery and armor battalions were followed by engineering units, logistic support units and communications units; headquarters were formed to coordinate their employment.