At the start of 1971 South Vietnamese troops continued operations against the North Vietnamese People's Army of Vietnam (PAVN) and Vietcong (VC) base areas in eastern Cambodia. The ill-conceived and poorly executed Operation Lam Son 719 against PAVN supply lines in eastern Laos showed the weaknesses within the South Vietnamese military command and the limited ability of South Vietnam's armed forces to conduct large-scale combined arms operations. The U.S. continued its unilateral withdrawal from South Vietnam despite the lack of any progress in the Paris Peace Talks and by November U.S. forces had ceased offensive operations. The U.S. withdrawal and antiwar sentiment within the military led to an ongoing decline in morale and discipline within the U.S. forces and growing drug use, particularly of heroin. As U.S. combat units withdrew, security in their former operational areas deteriorated and the PAVN/VC began a series of attacks on ARVN positions in Quảng Trị province and the Central Highlands. In Cambodia the Cambodian government continued to lose ground to the PAVN despite extensive U.S. air support and training and periodic attacks into Cambodia by the ARVN. While the bombing of North Vietnam had ceased in November 1968, U.S. aircraft continued to conduct reconnaissance flights over the North and responded to radar-tracking and antiaircraft fire with "protective reaction" strikes which numbered more than 100 by the year-end and culminated in a five-day bombing campaign in late December.
January 1 January U.S. military personnel in South Vietnam totaled 334,600 on 31 December 1970.
1 January - May 1971 Project Copper was an unsuccessful operation to use three Central Intelligence Agency (CIA)-trained Cambodian irregular force battalions to interdict the Sihanouk Trail. One battalion deserted, one mutinied during training and the third suffered extensive casualties and had to be withdrawn to assist in the defense of Phnom Penh.
5 January The United States Congress adopted the revised Cooper-Church Amendment which prohibited the introduction of U.S. ground troops or advisers into Cambodia and declared that U.S. aid to Cambodia should not be considered a commitment to the defense of Cambodia.CQ Almanac, accessed 10 Aug 2015
5 January - 11 February Operation Silver Buckle was a Royal Lao Army (RLA) offensive staged in Military Region 4 of Laos and was the deepest RLA penetration to date of the Ho Chi Minh Trail. Reaching the Trailside village of Moung Nong, the forward two companies attacked the rear of the 50,000 man People's Army of Vietnam (PAVN) garrison on 8 February 1971, just as Operation Lam Son 719 was launched by the Army of the Republic of Vietnam (ARVN) and diverted at least six PAVN battalions away from the ARVN assault.
5 January - 30 May The ARVN launched Operation Toàn Thắng TT02 which culminated in the Battle of Snuol against PAVN and Viet Cong (VC) forces in the Snuol District of Cambodia. The PAVN/VC lost 1,043 killed while the ARVN lost 37 killed, 74 missing and more than 300 captured. The operation rendered the ARVN 5th Division combat ineffective.
6 January U.S. Secretary of Defense Melvin Laird said that the "Vietnamization" of the war was running ahead of schedule and that the combat mission of the U.S. troops would end in summer 1971.
Military Assistance Command Vietnam (MACV) announced a program to combat drug abuse among U.S. servicemen in South Vietnam with some 65,000 servicemen estimated to be drug abusers in 1970.
7 January
The last herbicide spraying by the United States to defoliate forests in South Vietnam and kill crops used to feed communist soldiers and supporters was carried out in Ninh Thuan province. Operation Ranch Hand was finished.Buckingham, Jr., William A. (1982) Operation Ranch Hand: The U.S. Air Force and Herbicides, 1961-1971 Washington, D.C.: Office of Air Force History, p. 175
8 January U.S. jets carried out "protective reaction" strikes attacking two North Vietnamese Surface to air missile sites north of the Vietnamese Demilitarized Zone (DMZ).
11 January - 29 March The 1st Battalion 1st Marines conducted Operation Upshur Stream, continuous reconnaissance and infantry patrolling and concentrated air and artillery attacks in an effort to prevent the PAVN/VC from using the Charlie Ridge base area for attacks against Da Nang.
12 January A group of religious anti-war activists, led by Philip Berrigan, who became known as the Harrisburg Seven, were indicted on charges of planning to kidnap National Security Advisor Henry Kissinger and blow up steam tunnels under federal buildings in Washington D.C.
The Concerned Officers Movement held a press conference where they called for an investigation into the "responsibility for war crimes of key military figures", including Generals William Westmoreland and Creighton Abrams and Admiral Elmo Zumwalt.
13 January President Richard Nixon signed legislation repealing the Gulf of Tonkin Resolution which had been used as the legal basis for U.S., involvement in the war.
13 to 25 January The ARVN 4th Armor Brigade and 4th Ranger Group and the 2nd Marine Brigade together with Khmer National Armed Forces (FANK) forces launched Operation Cuu Long 44-02 to reopen Route 4 in Cambodia. The operation resulted in 211 PAVN and 16 ARVN killed.
17 January 300 ARVN paratroopers with U.S. air support and advisers raided a suspected camp holding American prisoners of war in Cambodia. No POWs were in the camp, but 30 PAVN soldiers were captured.
Joint U.S., South Vietnamese and Cambodian convoys begin up the Mekong River to supply Phnom Penh after Khmer Rouge attacks had cut off overland supply from Cambodia's seaports.
21-2 January PAVN sappers attacked Pochentong Airfield near Phnom Penh and destroyed or damaged 69 Khmer National Aviation (AVNK) aircraft and killed 39 AVNK personnel, effectively destroying the AVNK.
24 January NBC reported that soldiers of the 173rd Airborne Brigade stationed at Landing Zone English were buying heroin from a Vietnamese house on the base and the South Vietnamese then proceeded to demolish the house.
30 January U.S. forces launch Operation Dewey Canyon II to reopen Route 9 to the abandoned Khe Sanh Combat Base as the initial phase of Operation Lam Son 719.
February 2 February - 30 April Campaign 74B was a PAVN combined arms operation that recaptured the strategic Plain of Jars and brought the PAVN 316th Division within artillery range of the major RLA base at Long Tieng. The assault was stopped by Thai mercenary forces and U.S. air support and the PAVN withdrew as they exhausted their supplies.
3 February - 10 March In Operation Hoang Dien 103, units of III Marine Amphibious Force, Republic of Korea Marine Corps 2nd Marine Brigade, ARVN 51st Regiment, 146th Popular Force (PF) Platoon, 39th Regional Force (RF) Company and People's Self-Defense Force (PSDF) units combed the Da Nang lowlands and lowland fringes, killing 330 PAVN/VC, while losing 46 killed, including two Americans.
8 February President of the Khmer Republic (Cambodia) Lon Nol suffered a stroke and passed control to Prince Sisowath Sirik Matak.
8 February- 25 March Operation Lam Son 719 () was an invasion by 20,000 soldiers of the armed forces of South Vietnam of southeastern Laos. The objective of the operation was the disruption of the Ho Chi Minh Trail which supplied PAVN and VC forces in South Vietnam. Although claiming victory, the ARVN withdrew from Laos in disorder and suffered 9,000 casualties. U.S. forces were prohibited from entering Laos but provided air support for the operation and had 253 soldiers killed and many helicopters destroyed.
10 February In Operation Lam Son 719, an armoured column of the ARVN reached Ban Dong, 20 kilometers inside Laos and one half the distance to Tchepone, the objective of the invasion. The route, Highway 9, was only barely passable and the advance stalled. The PAVN concentrated their resistance against a number of small bases established in Laos to support the operation.
A Republic of Vietnam Air Force (RVNAF) UH-1 helicopter carrying photojournalists Larry Burrows, Henri Huet, Kent Potter and Keizaburo Shimamoto and seven others was shot down over Laos killing all on board.
12 February Armée Nationale Khmère (ANK) Brigadier General Neak Sam was killed along with 14 of his men during operations against the PAVN off Highway 2 south of Takéo.
13 February Lon Nol was flown to Hawaii aboard a United States Air Force (USAF) jet and taken to Tripler Army Medical Center for treatment.
15-17 February U.S. jets hit North Vietnamese SAM sites around Vinh over three consecutive days in "protective reaction" strikes.
16 February USAF Colonel Gerald V. Kehrli, the highest-ranking U.S. officer court-martialled in South Vietnam, was found guilty of seven marijuana offenses and sentenced to three years in prison and a $15,000 fine.
16 February - 3 April Operation Desert Rat was an RLA operation intended to harass the PAVN as they fought off the ARVN in Operation Lam Son 719. The operation resulted in 121 PAVN killed and 39 trucks destroyed.
17 February In his first press conference since the start of Lam Son 719, Nixon refused to put any limits on the use of U.S. airpower in Indochina other on than the use of tactical nuclear weapons. Nixon warned of stiffening PAVN resistance in Laos as the PAVN sought to defend their supply lines into South Vietnam and fuelled speculation that the South might attack into North Vietnam.
18 February North Vietnam said that the U.S. was threatening to expand the war into North Vietnam and that this also threatened China.
20 February ARVN forces abandoned Landing Zone (LZ) Ranger North () in Laos after two days of artillery bombardment and then armor-infantry assaults. ARVN losses were 178 dead or missing while an estimated 639 PAVN troops were killed during the battle.
22 February A story in The New York Times titled "Saigon's Rangers driven from an outpost in Laos" depicted the fight at Ranger North as a panicked debacle for the ARVN with able-bodied Rangers pushing their way onto medevac helicopters.
23 February New ARVN I Corps commander Lieutenant General Đỗ Cao Trí died in a helicopter crash near Bien Hoa Air Base. Photojournalist François Sully leapt from the burning helicopter but later died of his injuries.
25 February ARVN Airborne forces abandoned Fire Support Base (FSB) 31 () in Laos after two days of artillery bombardment and then armor-infantry assaults. PAVN losses were approximately 250 killed and 11 PT-76 and T-54 tanks destroyed. The Airborne had 155 killed and over 100 captured.
28 February The U.S. moved armored units to the Laos border to prevent any PAVN armored thrust eastwards after the loss of FSB 31.
March 1 March A bomb exploded in the United States Capitol building at 1:32 a.m., injuring nobody but causing $300,000 in damage. The Weather Underground took credit for the bombing which was in protest of the invasion of Laos.
2 March The ARVN abandoned FSB 30 () in Laos after heavy PAVN artillery fire and ground assaults.
A PAVN mortar and sapper attack on Cambodia's only oil refinery at Kompong Som killed five Cambodians and destroyed 80% of the facility, four PAVN were killed.
Eleven ARVN were killed in an ambush near Suong, Cambodia.
3 March The 5th Special Forces Group departed South Vietnam.
4 March North Vietnam fired three SA-2 missiles at U.S. jets flying over northern South Vietnam.
A U.S. Army U-21 on a radio research mission was shot down by a SAM near the DMZ killing all five crewmen on board.
In a press conference Nixon said that Operation Lam Son 719 had seriously damaged North Vietnam's military capabilities and would aid the U.S. withdrawal of forces.
5 March The 11th Armored Cavalry Regiment, less its 2nd Squadron departed South Vietnam.
6 March