This campaign was from 25 December 1965 to 30 June 1966. United States operations after 1 July 1966 were a continuation of the earlier counteroffensive campaign. Recognizing the interdependence of political, economic, sociological, and military factors, the Joint Chiefs of Staff declared that American military objectives should be to cause North Vietnam to cease its control and support of the insurgency in South Vietnam and Laos, to assist South Vietnam in defeating Viet Cong and North Vietnamese forces in South Vietnam, and to assist South Vietnam in pacification extending governmental control over its territory.
North Vietnam continued to build its own forces inside South Vietnam. At first this was done by continued infiltration by sea and along the Ho Chi Minh trail and then, in early 1966, through the Demilitarized Zone (DMZ). U.S. air elements received permission to conduct reconnaissance bombing raids, and tactical air strikes into North Vietnam just north of the DMZ, but ground forces were denied authority to conduct reconnaissance patrols in the northern portion of the DMZ and inside North Vietnam. Confined to South Vietnamese territory U.S. ground forces fought a war of attrition against the enemy, relying for a time on body counts as one standard indicator for measuring successful progress for winning the war.
During 1966 there were eighteen major operations, the most successful of these being Operation WHITE WING (MASHER). During this operation, the 1st Cavalry Division, Korean units, and ARVN forces cleared the northern half of Binh Dinh Province on the central coast. In the process they decimated a division, later designated the North Vietnamese 3d Division. The U.S. 3d Marine Division was moved into the area of the two northern provinces and in concert with South Vietnamese Army and other Marine Corps units, conducted Operation HASTINGS against enemy infiltrators across the DMZ.
The largest sweep of 1966 took place northwest of Saigon in Operation ATTLEBORO, involving 22,000 American and South Vietnamese troops pitted against the VC 9th Division and a NVA regiment. The Allies defeated the enemy and, in what became a frequent occurrence, forced him back to his havens in Cambodia or Laos.
By 31 December 1966, U.S. military personnel in South Vietnam numbered 385,300. Enemy forces also increased substantially, so that for the same period, total enemy strength was in excess of 282,000 in addition to an estimated 80,000 political cadres. By 30 June 1967, total U.S. forces in SVN had risen to 448,800, but enemy strength had increased as well.
On 8 January U.S. and South Vietnamese troops launched separate drives against two major VC strongholds in South Vietnam-in the so-called "Iron Triangle" about 25 miles northwest of Saigon. For years this area had been under development as a VC logistics base and headquarters to control enemy activity in and around Saigon. The Allies captured huge caches of rice and other foodstuffs, destroyed a mammoth system of tunnels, and seized documents of considerable intelligence value.
In February, the same U.S. forces that had cleared the "Iron Triangle", were committed with other units in the largest allied operation of the war to date, JUNCTION CITY. Over 22 U.S. and four ARVN battalions engaged the enemy, killing 2,728. After clearing this area, the Allies constructed three airfields; erected a bridge and fortified two camps in which CIDG garrisons remained as the other allied forces withdrew.