Air campaigns in Cambodia and Laos
U.S. military operations in Southeast Asia began earlier than often realized, just as it is little realized that the priority of the John F. Kennedy administration was Laos, not Vietnam. The first raids were small, although there were eventually huge B-52 area raids, as well as specialized attacks along the Ho Chi Minh trail. Other operations outside North and South Vietnam were conducted against targets in Cambodia.
Deniability and rules of engagement
Many of the missions in Laos and Cambodia, especially in the early days, were not acknowledged by the United States. Obviously, the enemy knew they were being attacked, but the U.S. political leadership wanted to deny the Vietnamese Communists the opportunity to claim that the U.S. was violating Geneva accords covering multiple nations in Southeast Asia. There was also a perceived need for face-saving on the part of Cambodian and Laotian government leaders, who did not want to admit that either Communist or non-Communist forces were operating in their countries.
"If I decide that there’s no way we can effect your rescue [in Cambodia], I’ll order the gunships to fire at you to prevent the enemy from getting their hands on you. I can’t risk having any of the [recon] teams compromised if they take you alive."
Limited air supply and other covert but noncombat missions were deployed in December 1960. Combat operations began covertly in 1961, starting with Operation FARM GATE. Later programs, divided by geographic area, included Operations BARREL ROLL, STEEL TIGER, TIGER HOUND, and COMMANDO HUNT. 
Covert operations, 1961
The U.S. Air Force created on April 14, 1961, the 4400th Combat Crew Training Squadron (CCTS), code named “Jungle Jim.” The unit, of about 350 men, had 16 C-47 transports, eight B-26 bombers, and eight T-28 trainers (equipped for ground attack), with an official of training indigenous air forces in counterinsurgency and conduct air operations. A volunteer unit, they would deploy in October, to begin FARM GATE missions.
"FARM GATE" was supposedly derived from the military slang expression for a soldier killed in battle: "He bought the farm".
In October, a U.S. Air Force special operations squadron,, part of the 4400th CCTS deployed to SVN, officially in a role of advising and training. The aircraft were painted in South Vietnamese colors, and the aircrew wore uniforms without insignia and without U.S. ID. Sending military forces to South Vietnam was a violation of the Geneva Accords of 1954, and the U.S. wanted plausible deniability.
The deployment package consisted of 155 airmen, eight T-28s, and four modified and redesignated SC-47s and subsequently received B-26s. U.S. personnel flew combat as long as a VNAF person was aboard. FARMGATE stayed covert until after the Gulf of Tonkin incident.
Early operations against the Ho Chi Minh Trail
Remote sensing, in the broadest sense, began with US operations against the Laotian part of the Ho Chi Minh trail, in 1961. Under CIA direction, Lao nationals were trained to observe and photograph traffic on the Trail . This produced quite limited results, and, in 1964, Project LEAPING LENA parachuted in teams of Vietnamese Montagnards led by Vietnamese Special Forces.The very limited results from LEAPING LENA led to two changes in 1965.
As opposed to other missions over Laos that targeted the Ho Chi Minh trail, Operation BARREL ROLL (1968-73) directly supported troops of the Royal Laotian Government (RLG), although it did have a secondary roll in interdicting the Trail.
Originally, it was constrained due to higher priorities for operations in North and South Vietnam.
- interdicting enemy supplies moving through northern Laos and
- providing air support for Laotian ground forces fighting the Pathet Lao and People's Army of Viet Nam
When Lyndon B. Johnson stopped the bombing of North Vietnam in Operation ROLLING THUNDER, many of the resources were shifted to Laos. Under the Joint Chiefs' criteria, Barrel Roll was an effective air campaign, more so than Operation ROLLING THUNDER. 
Later operations against the Trail
The U.S. ran two other air operations against the PAVN and Pathet Lao in Laos, STEEL TIGER, starting on April 3, 1965, of which a subset, TIGER HOUND, was put under MACV operational control on December 9. These primarily used fighter-bombers.
Unusual among regional projects, STEEL TIGER used carrier-based U.S. Navy aircraft as well as U.S. and South Vietnamese Air Force. STEEL TIGER reported not to MACV, but originally to the 2nd Air Division/2nd Air Force, which reported to United States Pacific Command. The reporting chain for Navy aircraft in STEEL TIGER was Task Force 77, the United States Seventh Fleet, and then Pacific Command.
MACV-SOG began to send in US-led reconnaissance teams. These Army teams worked closely with US Air Force forward air controllers (FAC). While the FACs immediately helped, air-ground cooperation improved significantly with the use of remote geophysical MASINT sensors, although MASINT had not yet been coined as a term. The original sensors, a dim ancestor of today's technologies, started with air-delivered sensors under Operation Igloo White, such as air-delivered Acoubuoy and Spikebuoy acoustic sensors . These cued monitoring aircraft, which sent the data to a processing center in Thailand, from which target information was sent to the DELTA teams. 
This was a reallocation of former ROLLING THUNDER aircraft to Laos, after the bombing halt of March 1958.
Support to MACV-SOG Covert Operations
Within MACV-SOG, two branches, internally designated OP32 (Air Studies Branch) and OP75 (air operations), drawn primarily from the Air Force but also other services, were involved with air operations. The cover name for the opertional unit was the 75th Air Studies Group. Its operations outside South Vietnam were primarily against North Vietnam, it flew forward air controller mission, under the call sign Covey, starting in 1965.  In addition, it did fly some MC-130 COMBAT SPEAR infiltration missions into Laos in 1970.. There were also helicopter infiltration and exfiltration missions, usually by Air Force but sometimes Army helicopter crews. 
While air operations in Laos went back to the Kennedy Administration, there were no combat actions at all under Johnson. MACV-SOG reconnaissance teams, under the code name DANIEL BOONE, did conduct surveillance. GEN Creighton Abrams, who had succeeded GEN William Westmoreland as the head of U.S. forces in South Vietnam, requested bombing of Cambodian sanctuaries in July 1968, but the first bombing took place in March 1969.
Some missions infiltrating and exfiltrating ground reconnaissance patrols, as well as Covey fAC missions were flown starting in 1965 and continuing into the 1970s. Until the announced U.S. border crossings, these were to be strictly deniable; it was these missions where pilots told that if they could not be recovered, they could be fired upon to prevent capture. 
Air attacks on PAVN bases near the Cambodian border started on March 18, 1969, under the overall name MENU, with individual base areas with code names such as SNACK and LUNCH. Approximately 120,000 tons of bombs were dropped, which presumably were noticed by surviving North Vietnamese. In the estimation of the Nixon Administration, which kept the information tightly held, the North Vietnamese would remain silent since they also maintained the fiction of Cambodian neutrality, important to Prince Norodom Sihanouk, the Cambodian head of state. Among the officials aware of it, there was dissent. Congress as a whole was not told about it, only Senators Richard Russell and John Stennis, the chairman and ranking minority members of the Senate Armed Services Committee. Strongly in favor were Abrams, Ambassador Ellsworth Bunker, Assistant to the President for National Security Affairs Henry Kissinger and his deputy Alexander Haig, and Chairman of the Joint Chiefs of Staf GEN Earle Wheeler. Secretary of Defense Melvin Laird approved of the operation but not the secrecy. Domestic policy probably also played a major role, as Nixon was concerned about the reaction of the antiwar movement, which was indeed strong when the operation became public.
Secretary of State William Rogers was concerned that it would jeopardize the Paris Peace Talks. Nixon personally argued for the operation, saying that military action was the only thing the North Vietnamese would understand. 
Operations become public
On March 18, 1970, Sihanouk, who had been traveling in Moscow, asking for Soviet intervention to oust the PAVN, was deposed by his prime minister, GEN Lon Nol, and the deputy prime minister, Prince Sirik Matak. The ostensible political reason to save face for Sihanouk was gone.  The fact of U.S. involvement in Cambodia was announced, on April 30, 1970. by Nixon, in the context of U.S. and South Vietnamese forces making a limited ground and air incursion into Cambodia. It was made into the areas called the Parrot's Beak and the Fish Hook, and was stated to be at the request of the new leaders of Cambodia, 
When the New York Times published a report on May 9, 1970, Nixon and Kissinger ordered warrantless surveillance on several White House staff members suspected of leaking the information. Since access to certain classified programs requiring special access may include preagreement for surveillance, it is possible that this may have been within the security agreements. That one of the staff under surveillance, Morton Halperin, resigned and later sued Kissinger does not suggest prior consent. In any event, according to William Beecher, the Times reporter, his source was a British reporter in Cambodia, not anyone on Nixon's staff.
Operation Lam Son 719
During the 1970 Vietnamization efforts, the ARVN launched Operation Lam Son 719, a ground operation into Cambodia, for which U.S. ground troops but not aircraft were excluded; the U.S. provided close air support as well as airlift.
Major B-52 strikes
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