Question: The McNamara Line was an operational strategy employed by the United States in 1966–1968 during the Vietnam War to prevent infiltration of South Vietnam by NVA forces from North Vietnam and Laos, the sophisticated electronic surveillance was backed with assorted types of mines, named "the barrier system" by Robert Strange McNamara, was an American business executive and the eighth Secretary of Defense, serving from 1961 to what year?

Context paragraphs:
Operation Steel Tiger: Operation "Steel Tiger" was a covert U.S. 2nd Air Division, later Seventh Air Force and U.S. Navy Task Force 77 aerial interdiction effort targeted against the infiltration of People's Army of Vietnam (PAVN) men and material moving south from the Democratic Republic of Vietnam (DRV or North Vietnam) through southeastern Laos to support their military effort in the Republic of Vietnam (RVN or South Vietnam) during the Vietnam War.

Operation Lam Son 719: Operation Lam Son 719 (Vietnamese: "Chiến dịch Lam Sơn 719 or Chiến dịch đường 9 – Nam Lào" ) was a limited-objective offensive campaign conducted in the southeastern portion of the Kingdom of Laos.  The campaign was carried out by the armed forces of the Republic of Vietnam (South Vietnam) between 8 February and 25 March 1971, during the Vietnam War.  The United States provided logistical, aerial, and artillery support to the operation, but its ground forces were prohibited by law from entering Laotian territory.  The objective of the campaign was the disruption of a possible future offensive by the People's Army of Vietnam (PAVN), whose logistical system within Laos was known as the Ho Chi Minh Trail (the Truong Son Road to North Vietnam).

1973 in the Vietnam War: 1973 in the Vietnam War began with a peace agreement, the Paris Peace Accords, signed by the United States and South Vietnam on one side of the Vietnam War and communist North Vietnam and the insurgent Viet Cong on the other.  Although honored in some respects, the peace agreement was violated by both North and South Vietnam as the struggle for power and control of territory in South Vietnam continued.  North Vietnam released all American prisoners of war and the United States completed its military withdrawal from South Vietnam.

McNamara Line: The McNamara Line was an operational strategy employed by the United States in 1966–1968 during the Vietnam War to prevent infiltration of South Vietnam by NVA forces from North Vietnam and Laos.  The McNamara Line ran across South Vietnam from the South China Sea to the Laotian border along the Vietnamese Demilitarized Zone (DMZ).  The eastern part included fortified field segments with Khe Sanh as linchpin, along with stretches, where roads and trails were guarded by the high-tech acoustic and heat-detecting sensors on the ground and interdicted from the air.  A sophisticated electronic surveillance was backed with assorted types of mines, including so-called gravel mines, and troops at choke points.  Named "the barrier system" by Robert McNamara, it was one of the key elements, along with gradual aerial bombing, of his war strategy in Vietnam.

Ho Chi Minh trail: The Hồ Chí Minh trail (also known in Vietnam as the "Trường Sơn trail") was a logistical system that ran from the Democratic Republic of Vietnam (North Vietnam) to the Republic of Vietnam (South Vietnam) through the kingdoms of Laos and Cambodia.  The system provided support, in the form of manpower and materiel, to the National Front for the Liberation of South Vietnam (called the Viet Cong or "VC" by its opponents) and the People's Army of Vietnam (PAVN), or North Vietnamese Army, during the Vietnam War.

Project Dye Marker: Project Dye Marker was a cover name for a strong point/obstacle component of the electronic anti-infiltration barrier system in South Vietnam known as "McNamara Line" during the Vietnam War, which aimed to create an alternative to the US bombing of North Vietnam.  The fortifications were partially constructed by the American forces in 1967-1968 along the eastern portion of the demilitarized zone.  An effective anti-infiltration barrier, running across South Vietnam deep into Laos, was a grand vision of the US Secretary of Defence Robert McNamara, who feared that escalation of bombing can bring greater Chinese involvement, 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.

North Vietnamese invasion of Laos: North Vietnam supported the Pathet Lao to fight against the Kingdom of Laos between 1958–1959.  Control over Laos allowed for the eventual construction of the Ho Chi Minh Trail that would serve as the main supply route (MSR) for enhanced NLF (the National Liberation Front, the Vietcong) and NVA (North Vietnamese Army) activities in the Republic of Vietnam.  As such, the support for Pathet Lao to fight against Kingdom of Laos by North Vietnam would prove decisive in the eventual communist victory over South Vietnam in 1975 as the South Vietnamese and American forces could have prevented any NVA and NLF deployment and resupply if these only happened over the 17th Parallel, also known as the Demilitarized Zone (DMZ), a narrow strip of land between North and South Vietnam that was closely guarded by both sides.  It also helped the Pathet Lao win the Kingdom of Laos, although the Kingdom of Laos had American support.

Vadim Shcherbakov: Vadim Petrovich Shcherbakov was an officer in the Soviet Air Defense Forces (PVO Strany).  As a senior lieutenant, he was serving in the 260th "Bryansky" Anti-Aircraft Missile Regiment.  In 1966-1967 Shcherbakov's whole regiment was temporarily assigned as a Group of Soviet Military Advisors in Vietnam () and in March 1966 was transferred to North Vietnam, where the regiment was tasked with establishing a training center and training the NVA's 274th Anti-Aircraft Missile Regiment for combat duty.  In North Vietnam Shcherbakov served as a SAM operator/instructor, and specialized as a fire control operator during combat deployments.  For his service Shcherbakov received the Order of Lenin, the highest Soviet decoration after returning from North Vietnam in 1967.  in their 18th report claims that formerly closed soviet sources credited Col Shchbakov with 10 combat engagements and 6 kills.  After Vietnam, Shcherbakov continued his service, eventually reaching the rank of Colonel.  Very little is known about his biography except for multiple brief mentions in Soviet Officers' Vietnam War memoirs.  He was still alive in 1991, when he was attending an annual meeting of the Russian Association of Vietnam War veterans.  He was a retired Colonel at that time.

Robert McNamara: Robert Strange McNamara (June 9, 1916 – July 6, 2009) was an American business executive and the eighth Secretary of Defense, serving from 1961 to 1968 under Presidents John F. Kennedy and Lyndon B. Johnson.  He played a major role in escalating the United States involvement in the Vietnam War.  McNamara was responsible for the institution of systems analysis in public policy, which developed into the discipline known today as policy analysis.

1965 in the Vietnam War: In 1965, the United States rapidly increased its military forces in South Vietnam, prompted by the realization that the South Vietnamese government was losing the Vietnam War as the communist-dominated Viet Cong gained influence over much of the population in rural areas of the country.  North Vietnam also rapidly increased its infiltration of men and supplies to combat South Vietnam and the U.S.  The objective of the U.S. and South Vietnam was to prevent a communist take-over.  North Vietnam and the insurgent Viet Cong sought to unite the two sections of the country.

Supporting paragraphs: McNamara Line Robert McNamara
Answer: 1968