At the beginning of 1966, the number of U.S. military personnel in South Vietnam totaled 184,300. South Vietnamese military forces totaled 514,000 including the army (ARVN) and the Regional Force and Popular Force (the "Ruff-Puffs") militias. The North Vietnamese People's Army of Vietnam (PAVN) numbered 400,000, most still in North Vietnam. 50,000 PAVN cadre and soldiers infiltrated South Vietnam during 1965. Group 559, charged with transporting supplies down the Ho Chi Minh Trail to supply PAVN troops in both South Vietnam and Laos, numbered 24,400 personnel. The U.S. estimated the number of Viet Cong (VC) and PAVN soldiers in South Vietnam at nearly 280,000 by June 1966, including part-time guerrillas. A pause in the bombing of North Vietnam by U.S. warplanes had been announced by President Johnson on 24 December and remained in effect.

January Writing in Harper's Magazine retired U.S. general James M. Gavin proposed an enclave strategy where U.S. forces would defend coastal enclaves where most of the population and economic activity was concentrated and act as a strategic reserve, leaving the ARVN to fight the PAVN/VC further inland. This strategy was designed not to achieve victory, but rather to create a stalemate forcing the PAVN/VC to seek a negotiated solution. In later years the "Gavin Plan" would become known as "light at the top, heavy at the bottom" reflecting the population concentration of South Vietnam.

"The Ballad of the Green Berets" by Staff sergeant Barry Sadler is released. It became the No. 1 hit in the U.S. for the five weeks spanning March 1966.

1–8 January Operation Marauder was conducted by the U.S. 173rd Airborne Brigade and the 1st Battalion, Royal Australian Regiment (1 RAR) in the Plain of Reeds, Mekong Delta. The operation resulted in 114 VC and three U.S. killed.

3 January The PAVN bombarded a Special Forces Civilian Irregular Defense Group camp at Khe Sanh Combat Base near the Vietnamese Demilitarized Zone (DMZ) with 120mm mortars, the heaviest weapon they had used in the war. Defending the combat base were American and South Vietnamese Special Forces, Nung and Bru (Montagnard) irregulars and militia forces.Prados, John (2004), Valley of Death: The Siege of Khe Sanh, Annapolis: US Naval Institute Press, pp 37-44.

In his message to the Tricontinental Conference, Che Guevara called for creating "two, three many Vietnams." to fight imperialism in the southern hemisphere.

6 January The Student Nonviolent Coordinating Committee (SNCC) became the first African-American civil rights organization to publicly oppose the war.

7 January Commander of Military Assistance Command, Vietnam General William Westmoreland is Time magazine's Man of the year.

A Claymore mine exploded at Tan Son Nhut International Airport gate, killing two persons and injuring 12.

8–14 January Operation Crimp was a joint US-Australian military operation in the Ho Bo Woods, north of Cu Chi in Binh Duong Province, about north-east of Saigon. The operation was conducted by the U.S. 1st Infantry Division and 173rd Airborne Brigade and 1 RAR resulted in 128 VC killed and 92 captured and 14 U.S. and 8 Australians killed.

9-11 January Operation Flying Tiger VI was an ARVN and ROK Capital Division search and destroy operation in Bình Định Province. The operation resulted in 192 VC and 11 ARVN/ROK killed.

12 January In his State of the Union Address President Lyndon Johnson told Congress and television viewers that the nation could afford both the funding of the cost of social programs and an ongoing war, saying "I believe that we can continue the Great Society while we fight in Vietnam."

15 January - 25 February

Operation Van Buren was a harvest security operation conducted by the 1st Brigade, 101st Airborne Division and the South Korean 2nd Marine Brigade in the Tuy Hòa Valley, Phú Yên Province. The operation resulted in 346 PAVN killed and 33 captured, 55 U.S. and 45 Koreans killed.

17 January VC in Kien Tuong Province detonated a mine under a highway bus, killing 26 civilians, seven of them children. Eight persons were injured and three missing.

25 January A USAF C-123K lost power and crashed after take-off from An Khe, en route to Bong Son killing all 46 persons on board.

26 January In a meeting with Johnson Republican senator John Sherman Cooper urged him to forgo his announced plan to resume bombing of North Vietnam and negotiate a settlement instead.

28 January The U.S. Selective Service System announced that it would change its guidelines for conscription of college students and college-bound high school graduates, by barring "Class 2-S" draft deferments for students whose grades were in the lower half of their freshman class, the lower one-third of their sophomore class or the lower one-fourth of their junior class.

28 January - 17 February Operation Double Eagle was conducted by U.S. 1st Marine Division and ARVN 2nd Division in Quảng Ngãi Province. The operation resulted in 312 VC killed and 19 captured and 24 Marines killed.

28 January - 6 March

Operation Masher was a combined U.S., ARVN and Republic of Korea Army operation in Bình Định Province conducted by the U.S. 1st Cavalry Division, ARVN 22nd Division and ROK Capital Division. The name "Operation Masher" was changed to "Operation White Wing", because Masher was deemed too crude for 'nation-building' by the White House. The operation resulted in 2,150 PAVN/VC, 288 U.S. and 10 Koreans killed. Masher failed to result in any decisive victories by the allies, but temporarily disrupted PAVN/VC control of the rural areas of the province. Masher and subsequent operations in Binh Dinh created large numbers of refugees who fled their homes to escape the fighting.

29 January The VC killed a Catholic priest, Father Phan Khac Dau, 74, at Thanh Tri, Kien Tuong province. Five other civilians, including a church officer, were also killed.

31 January After a 37-day moratorium that had started on 24 December 1965, the U.S. resumed Operation Rolling Thunder, the bombing of North Vietnam. Among the first targets destroyed were a bridge at Đồng Hới, a highway ferry complex in Thanh Hóa Province and barges near the city of Vinh.

February 2 February At Belmore Park in Sydney, three young Australian men became the first persons to burn their draft registration cards as a protest against Australia's participation in the war.

Special Forces Master Sergeant Donald W. Duncan appeared on the cover of Ramparts under the caption "I quit". He was one of the first service members to speak out against the war.

A VC squad ambushed a jeep of South Vietnamese information workers, killing six and wounding one in Hậu Nghĩa Province.

4 February Senator J. William Fulbright, chairman of the Senate Committee on Foreign Relations began a series of hearings on the war. Johnson took Fulbright's criticism of the war as a personal betrayal by his former friend.

6 February to 6 March Operation Thừa Thiên 177 was an ARVN 1st Division operation in Thừa Thiên Province. The operation resulted in 183 PAVN/VC and 17 ARVN killed.

7 February

In an apparent attempt to upstage the Fulbright Hearings, Johnson and Premier Nguyễn Cao Kỳ of South Vietnam convened with other officials at Camp H. M. Smith in Honolulu, Hawaii to discuss the course of the war.

Television was broadcast in South Vietnam for the first time, as the United States Navy used Stratovision sending a C-121 Constellation to carry transmitting equipment, videotape machines and a small television studio aloft. The C-121 took off from Tan Son Nhut Air Base, climbed to , then flew in a slow oval pattern at and, at 19:30, transmitted the first THVN programs to outdoor television sets that had been tuned to Channel 9.

7 February - May 1972 The USAF begins Operation Shed Light to develop night and adverse weather strike capabilities to interdict PAVN/VC operations.

10 February The People's Republic of China (PRC) accused the Soviet Union of forcing North Vietnam into peace negotiations with the U.S.

13 February A USAF Firebee 147E unmanned aircraft with electronic intelligence monitors was sent on a one-way mission to be shot down by the SA-2 antiaircraft radar and missile defense system being used by North Vietnam. The drone was picked up by the radar and destroyed, but not before "finally acquiring the long-mysterious command uplink and downlink signals" that were used in the SA-2 operation and relaying the data back to a nearby DC-130 aircraft; acquisition of the signal led to developing methods to jam it as well.

Operation An Dan 14/66 was an ARVN operation in Long An Province. the operation resulted in 150 VC and 21 ARVN killed.

14 February Two mines exploded beneath a bus and a three-wheeled taxi on a road near Tuy Hòa, killing 48 farm laborers and injuring seven others.

15-21 February Operation Thang Long 234 was an ARVN operation in Darlac Province. The operation resulted in 270 VC and 62 ARVN killed.

16-19 February In the First Battle of Nakhang the PAVN attacked the Royal Lao Army (RLA) garrison at Lima Site 36 in Na Khang. The attack was successful but U.S. air support inflicted heavy losses on the PAVN.

17 February The "Three-point Proposal" for ending the war was presented at the United Nations headquarters in New York by a spokesman for Secretary-General U Thant, calling for cessation of bombing of North Vietnam by the United States, a scaling down of military activities and an agreement by all sides to enter into discussions with representatives of the VC.M. S. Rajan and T. Israel, "The United Nations and the Conflict in Vietnam", in The Vietnam War and International Law, Volume 4: The Concluding Phase (Princeton University Press, 2015) p124

18 February The PRC consulate in Phong Saly, Laos, was heavily strafed by gunfire and the Beijing government charged that four American fighter jets had attacked "with more than 600 bullets", as well as dropping eight bombs to the east of the city, from the border with China.

Second Lieutenant Carol Ann Drazba and Second Lieutenant Elizabeth Ann Jones were among seven killed in a helicopter crash northeast of Tan Son Nhut Air Base. They were the first of eight women in the U.S. military killed in Vietnam.

19 February U.S. Senator Robert F. Kennedy became the first member of the Senate to break with President Johnson in proposing that the VC be allowed "a share of power and responsibility" in peace talks with the United States.

19 February to 1 March Operation Double Eagle II was conducted by the BLT 2nd Battalion, 3rd Marines, 2nd Battalion, 9th Marines and 2nd Battalion, 7th Marines in the Quế Sơn Valley. The operation resulted in 125 VC killed and ten captured for the loss of 6 Marines killed.

21-5 February Operation Mastiff was conducted by the U.S. 1st Infantry Division in Dầu Tiếng District against the PAVN 9th Division. The operation resulted in 61 PAVN and 17 U.S. killed.

22 February to 2 March Operation Lam Son 235 was a search and destroy operation conducted by two regiments of the ARVN 1st Division in Quảng Trị Province. The operation resulted in 444 PAVN/VC and 35 ARVN killed.

23-4 February The Battle of Suoi Bong Trang was fought on the night of 23–24 February 1966 between the U.S. 1st Brigade, 1st Infantry Division and Australian 1 RAR and PAVN/VC. The battle occurred during Operation Rolling Stone, a major American security operation to protect engineers building a tactically important road in the vicinity of Tan Binh, in central Binh Duong Province, northwest of Bien Hoa Air Base. The battle resulted 154 PAVN/VC killed and 15 captured and 11 U.S. killed.

23 February - 29 March The 1st Marine Division deployed to South Vietnam, establishing its headquarters at Chu Lai Base Area.

25 February to 24 March Operation Garfield was conducted by the 3rd Brigade, 25th Infantry Division in Darlac Province. The operation resulted in 122 PAVN/VC and 21 U.S. killed.

26 February - 3 March