The McNamara Line was an operational strategy employed by the United States in 1966–1968 during the Vietnam War, aimed to prevent infiltration of South Vietnam by People's Army of Vietnam (PAVN) forces from North Vietnam and Laos. Physically, the McNamara Line was to run across South Vietnam along the Vietnamese Demilitarized Zone (DMZ) from Cửa Việt to the Laotian border at Mường Phìn, Laos. The line included fortified bases along with stretches where roads and trails were guarded by high-tech acoustic and heat-detecting sensors on the ground and interdicted from the air. Assorted types of mines, including so-called gravel mines, and troops at choke points backed sophisticated electronic surveillance. Named the barrier system by Robert McNamara (United States Secretary of Defense from 1961 to 1968), it was one of the key elements, along with gradual aerial bombing, of his war strategy in Vietnam.

Various schemes had been proposed in the years before 1965 for a defensive line on the northern border of South Vietnam and in southeast Laos. These schemes had generally been rejected because of their requirements for large amounts of military personnel to be deployed in static positions and because any barrier in Laos would encourage the Vietnamese to deploy their forces deeper into Laotian territory.

In December 1965, Robert McNamara met twice with Carl Kaysen, a former Kennedy-era National Security Council staff member. Kaysen proposed an electronic barrier to limit infiltration from North Vietnam. McNamara embraced the idea and asked Kaysen to create a proposal. Starting in January, John McNaughton and a group of scientists in Cambridge, Massachusetts, including Kaysen and Roger Fisher created the proposal which was submitted to McNamara in March 1966, who then passed it to the Joint Chiefs of Staff (JCS) for comments. The JCS response was that the proposal would still require an infeasible number of troops to be stationed along the barrier and would present difficult construction/logistical problems.

Also in late 1965 or early 1966, Jerry Wiesner and George Kistiakowsky persuaded McNamara to support a summer study program in Cambridge for the group of 47 prominent scientists and academics that made up the JASON advisory division of the Institute for Defense Analysis. The subject of the study was to find alternatives to the majorly unsuccessful gradual aerial bombing campaign in North Vietnam advocated by McNamara. As Kaysen and the others involved in the Cambridge group were all members of JASON scientific advisory group, the anti-infiltration barrier ideas were included in the JASON agenda.

The JASON study group meetings took place June 16–25, 1966 at Dana Hall in Wellesley, Massachusetts. The buildings were guarded day and night and attendees were given top secret security clearances. After the summer meetings, a report was produced over the course of July and August.

The JASON report of August 1966 called the bombing campaign against North Vietnam a failure, saying that it had "no measurable direct effect on Hanoi's ability to mount and support military operations in the South". Instead, advisors proposed as an alternative two defensive barriers. The first barrier would run from the coast some distance inland along the DMZ and would seek to block PAVN infiltration through conventional means. The second barrier would run from the remote western areas of the border into Laos and would be a barrier of air interdiction, mine fields and electronic detection requiring minimal troops. While the JCS report had estimated the construction of a barrier would take up to four years, the JASON report suggested the barrier could be in place with available resources within a year. That was important to McNamara since he hoped that by cutting the logistics lines between the North and the South he would have been able to press Hanoi into negotiations.

In September 1966, McNamara presented the JASON group report to the JCS. It split on the proposal with the service chiefs being against it, and general Earle Wheeler, the chairman of JCS, being in support. The JCS then handed the report off to the Commander-in-Chief, Pacific (CINCPAC) Admiral Sharp, who wrote back that the barrier idea was impractical from a manpower and construction point of view. COMUSMACV General William Westmoreland, was apprehensive of the idea and reportedly was even afraid that the barrier would go into history as Westmoreland's Folly.

Despite all disagreements, on 15 September 1966, without waiting for the final judgment of the JCS, McNamara ordered that the proposal be implemented. Lieutenant General Alfred Starbird, director of the Defense Communications Agency, was appointed head of Task Force 728, which was to implement the project. Two days later, the JCS reported back favorably on the already-decided plan. Starbird had to complete the barrier by September 1967. In November 1966, McNamara officially recommended the barrier system to President Johnson for implementation. The construction budget was estimated as $1.5 billion, and $740 million was allocated for the annual operating costs. The Practice Nine was adopted as the barrier project internal communication code name. and a vital component of his operational strategy. It was an enormous multimillion project, which was nicknamed in the media as the Great Wall of Vietnam, McNamara's Wall, McNamara Barrier, Electric Fence, and Alarm Belt.