The United States continued its unilateral withdrawal of forces from South Vietnam notwithstanding the lack of progress at the Paris Peace Talks. The removal of Prince Norodom Sihanouk from power in Cambodia in March and his replacement by General Lon Nol, began the Cambodian Civil War. South Vietnamese and U.S. forces entered Cambodia in late April to attack People's Army of Vietnam (PAVN) and Vietcong (VC) bases and supply lines there which had long been used to support the insurgency in South Vietnam. The expansion of the war revitalized the antiwar movement in the U.S. and led to the Kent State shootings and Jackson State killings in May. While U.S. ground forces withdrew from Cambodia at the end of June and legislation was passed to prevent their reintroduction, the South Vietnamese conducted operations in Cambodia for the rest of the year and the U.S. provided air support and military aid to the Cambodian government. Despite this support the Cambodians lost control of vast areas of the country to the PAVN. Within South Vietnam the second half of the year saw a reduction in large U.S. operations with the focus shifting to pacification and population security and supporting Vietnamization. The PAVN/VC generally reverted to sapper attacks and attacks by fire but they fought hard to defend their base areas and infiltration routes.
January 2-3 January People's Army of Vietnam (PAVN) forces attacked a U.S. 23rd Infantry Division night defensive position near Đức Phổ Base Camp. In the four hour battle 29 PAVN and seven U.S. soldiers were killed.
6 January Under the cover of monsoon rains the Vietcong (VC) 409th Battalion attacked the 1st Battalion, 7th Marines at Firebase Ross with mortars and sappers penetrating the wire. The attack was repulsed by 04:00 with 13 Marines and 38 VC killed.
8 January President of South Vietnam Nguyễn Văn Thiệu announced that it would be impossible for U.S. forces to fully withdraw from South Vietnam by the end of 1970 and called for increased aid and training and phased U.S. withdrawal over several years.
16 January The VC attacked a refugee camp on the Batangan Peninsula killing 16 civilians and wounding 21.
18 January VC bombs exploded at the Thủ Đức Military Academy killing 18 people and wounding 33.
18-24 January The first Tour de Vietnam cycle race in 14 years began, following a course from Nha Trang to Long Xuyên. The race was to demonstrate the extent of government control over the countryside, however the competitors were escorted by armored vehicles and helicopter gunships.
19 January to 22 July The 1st Brigade, 5th Infantry Division launched Operation Green River in Quảng Trị Province. The operation results in 106 PAVN soldiers killed and 37 U.S. killed.
21 January A PAVN/VC artillery attack on Bien Hoa Air Base damaged a C–123, a C–7 and a UH–1.
A box of 81 mm mortar ammunition exploded at Firebase Nancy, north-northeast of Xuân Lộc, killed 13 soldiers of the U.S. 4th Battalion, 12th Infantry Regiment.
22 January A PAVN/VC attack on a Republic of Vietnam Marine Division (VNMC) base in the Mekong Delta results in 72 PAVN/VC and 15 VNMC killed.
28 January United States Air Force (USAF) jets attacked an SA-2 missile site near the Mụ Giạ Pass after it fired on a U.S. reconnaissance jet. One USAF F-105G Wild Weasel was shot down during the raid. Jolly Green 71 an HH-53B combat search and rescue helicopter was shot down by a missile fired from a Mig-21 piloted by Vu Ngoc Dinh of the Vietnam People's Air Force 921st Fighter Regiment as it attempted to rescue the aircrew. The 6-man crew were all killed.
A Gallup poll showed that 65% of Americans approved of President Richard Nixon's handling of the war.
30 January Nixon stated that Vietnamization was "irreversible" but that he would respond forcefully to any increase in North Vietnamese military activity.
February 3 February The United States Senate Committee on Foreign Relations led by Democratic Senator J. William Fulbright reopened hearings on the war. Republican Senator Charles Goodell described Vietnamization as "a great public relations success" but "not a true policy of disengagement."
5 February Starting at 18:00, Allied forces began a 24-hour Tết ceasefire.
U.S. jets attacked North Vietnamese antiaircraft positions in the Ban Karai Pass after they had fired on U.S. reconnaissance jets.
U.S. helicopter gunships accidentally attacked South Vietnamese Regional Forces test-firing a machine-gun killing eight and wounding 31.
10 February United States Secretary of Defense Melvin Laird in Saigon for a fact-finding mission said that he was satisfied with the current pace of Vietnamization.
12 February The PAVN 31st Regiment ambushed a 1st Battalion, 7th Marines patrol near Firebase Ross in the Quế Sơn valley. The U.S. 3rd Battalion, 21st Infantry Regiment was sent to support the Marines. U.S. losses were 13 killed.
14 February A Gallup poll showed that 55% of Americans opposed an immediate withdrawal from South Vietnam compared to 35% in favor.
16 February Two ARVN Ranger battalions and the 1st Armored Brigade reported killing 280 PAVN and capturing 36 after a series of encounters over five days south of Da Nang. ARVN losses were 23 killed.
17 February Military Policemen were called to a murder scene in an apartment at Fort Bragg. Special Forces Captain Jeffrey R. MacDonald claimed that four intruders attacked his family killing his pregnant wife and two young daughters. The case remains one of the most litigated murder cases in American criminal history.
Testifying before the Senate Foreign Relations Committee, William Colby, head of Civil Operations and Revolutionary Development Support (CORDS) denied that the Phoenix Program was an assassination program, while acknowledging that "some illegal killings" had taken place. Colby stated that in 1969 19,534 suspected VC cadres had been "neutralized" of which 6,187 had been killed, 8,515 captured and 4,832 defected. Colby said "I am neither optimistic nor pessimistic about the future of this program or Vietnam."
18 February Sergeant Michael Mullen was killed by shrapnel from friendly artillery fire while serving with the 6th Infantry Regiment in Quảng Tín Province. His death prompted his mother Peg Mullen to become an antiwar activist who was profiled by journalist C.D.B. Bryan in The New Yorker in 1976 and the story was subsequently made into the 1979 telemovie Friendly Fire.
USAF B-52 bombers were reported to be bombing PAVN and Pathet Lao forces on the Plain of Jars, the first time the heavy bombers had been used in northern Laos.
19 February In the Son Thang massacre five U.S. Marines killed five Vietnamese women and 11 children in Sơn Thắng, Quế Sơn Valley. Four of the Marines were charged with murder, two were acquitted and two convicted but only served sentences of one year.
21 February Despite the U.S. bombing, PAVN and Pathet Lao forces captured Xiangkhouang airfield and most of the Plain of Jars while the Royal Lao Army forces retreated west.
PAVN forces ambushed a mechanized infantry unit from the 196th Light Infantry Brigade in the Quế Sơn valley, killing 14 and wounding 29 in a five hour battle. PAVN losses were four killed.
27 February A PAVN/VC rocket attack on Bien Hoa Air Base damaged three A-37s, two F-100s and a C-7. Two USAF personnel were killed in these attacks and 74 wounded.
March 9 March Operational control of U.S. forces in I Corps passed from III Marine Amphibious Corps to XXIV Corps.
11 March Anti-Vietnamese demonstrations erupted in Phnom Penh, Cambodia while head of state Prince Norodom Sihanouk was touring Europe, the Soviet Union and China. Crowds attacked the North Vietnamese and VC embassies.
12 March Cambodian prime minister Lon Nol closed the port of Sihanoukville to the North Vietnamese and demanded that all PAVN and VC forces withdraw from Cambodian soil within 72 hours (on 15 March) or face military action.
14 March The first FTA Show is performed in Fayetteville, North Carolina, near Fort Bragg.
14 March – 12 April In the SS Columbia Eagle incident, two crewmembers seized the with the threat of a bomb and handgun and forced the captain to sail towards Cambodia. Twenty-four of the crew were forced to abandon the ship in lifeboats, while the remaining 13 sailed the ship into Cambodian waters and the two mutineers Clyde McKay Jr. and Alvin Glatkowski surrendered to the Cambodia government which gave them asylum. The mutineers were later arrested and imprisoned in Cambodia, but released later in the year. Glatowski eventually turned himself in to the U.S. Embassy, was extradited to the U.S. and served seven years in prison, while McKay sought refuge with the Khmer Rouge who apparently killed him.
16 March A U.S. Navy EC-121 Warning Star crashed on landing at Da Nang Air Base. The aircraft struck a hangar and caught fire killing 22 of the 31-man crew.
17 March The Peers Commission presented their report on the Mỹ Lai massacre. The report found that members of the 23rd Infantry Division had killed at least 175–200 Vietnamese men, women and children and that senior officers at brigade and divisional levels had participated in covering-up the incident.
18 March The Cambodian National Assembly voted to remove Sihanouk from power with Lon Nol taking the powers of the Head of State on an emergency basis.
19-30 March Fearing a joint ARVN–Cambodian attack Central Office for South Vietnam began moving from the Cambodia-South Vietnam border area into Kratié Province, Cambodia.
20 March The PAVN attacked Firebase Granite, the attack was repulsed with 10 U.S. and 10 PAVN killed.
21 March North Vietnamese Prime Minister Phạm Văn Đồng flew to Beijing to meet with Zhou Enlai and Sihanouk. Sihanouk agreed to accept the leadership of anti-Lon Nol forces under the umbrella of the National United Front of Kampuchea (FUNK).
22 March The VC bombed a Buddhist meeting in Hóc Môn District killing 14 civilians and wounding 20.
23 March
Sihanouk on Beijing radio called for a general uprising against Lon Nol and the formation of FUNK.
25 March North Vietnam withdrew its diplomats from Cambodia.
26 March North Vietnam refused an offer by South Vietnam for the release and repatriation of 343 wounded or ill prisoners of war, declaring that there were no members of the PAVN in the south. The North Vietnamese representatives at the Paris Peace Talks asserted that the captives were, instead, "illegally arrested patriots" from among South Vietnamese citizens rebelling against the Saigon government.
The South Vietnamese Government began the Land to the Tiller program of land reform designed to redistribute land to tenant farmers and undermine support for the VC.
27-8 March ARVN Rangers with armored cavalry support and support from U.S. helicopter gunships made the first large-scale South Vietnamese attack into Cambodia attacking a PAVN base in Kandal Province, killing 53 PAVN and capturing one for the loss of three ARVN killed.
28 March The White House announced that based on the judgment of commanders in the field, U.S. forces could cross into Cambodia in response to PAVN/VC threats. The White House denied that this represented any change in policy or widening of the war.
A U.S. Navy F-4 Phantom II shot down a VPAF MiG-21 while escorting a reconnaissance jet near Thanh Hóa in the first aerial engagement since the November 1968 bombing halt.
29 March The PAVN 272nd Regiment hit Firebase Jay with mortar, rocket and recoilless rifle fire hitting the command post and knocking out communications. The base was occupied by Companies A and E, 2nd Battalion, 7th Cavalry, B Battery, 2nd Battalion, 12th Artillery and B Battery, 2nd Battalion, 19th Artillery. The PAVN then launched infantry attacks on the base perimeter but were beaten back by dawn. U.S. losses were 13 killed, while 74 PAVN dead were found in and around the base.