The United States Air Force (USAF) saw Phou Pha Thi as an ideal location for installing a radar navigation system to assist pilots in their bombing campaigns in North Vietnam, and along the Ho Chi Minh Trail inside Laos.

When the plan to install a navigation system on Phou Pha Thi Mountain was proposed, Sullivan initially opposed it as he thought that Laotian Prime Minister Prince Souvanna Phouma would not allow his country to be involved in an aerial offensive against North Vietnam.

[2] In August 1966, the USAF installed a TACAN System, an autonomous radio transmitter that provided pilots and navigators with distance and bearing relative to the station on Phou Pha Thi.

In 1967, under the code name Heavy Green, the facility was upgraded with the TSQ-81, which could direct and control attacking jet fighters and bombers to their targets and provide them with precise bomb release points.

[5] As USAF ground controllers were able to guide attacking aircraft against North Vietnamese targets in all types of weather, installation of the TSQ-81 radar system on Phou Pha Thi was considered to have been extremely successful during the final months of 1967.

[7] Secord's fears were justified, as USAF reconnaissance aircraft regularly flying over northeastern Laos in 1967 revealed that the paved roads constructed by the North Vietnamese were obviously approaching Phou Pha Thi.

Road construction activities were observed along Routes 6 and 19, which connected Dien Bien Phu in North Vietnam with Phou Pha Thi and Nam Bac in Laos.

[6] Toward the end of 1967, US controllers at Lima Site 85 directed F-4, F-105 and A-1 fighter-bombers based in Thailand and South Vietnam in air strikes against North Vietnamese and Pathet Lao formations that were massing around the US facility at Phou Pha Thi.

On 14 January 1968, the situation in northeastern Laos continued to worsen, as an estimated four PAVN infantry battalions captured the Laotian government's stronghold at Nam Bac.

Despite the growing threat from North Vietnamese forces, the US military was still not permitted to reinforce the installation on Phou Pha Thi Mountain due to political sensitivities.

[8] The defense of Lima Site 85 was assigned to two CIA paramilitary officers who led about 1,000 Hmong soldiers, with 200 men guarding the ridge line and the remaining 800 in the valley below.

[11] Shortly afterwards, what remained of one of the An-2 biplanes was put on display in front of the That Luang Monument, Vientiane's most important Buddhist shrine, as proof of North Vietnamese military activities in the kingdom.

Throughout January and February, intelligence collected by the Hmongs confirmed that a major assault on Lima Site 85 was being prepared, but Sullivan and the US military took no steps to strengthen the defenses.

On 18 December 1967, following their intensive training, soldiers of the PAVN 41st Special Forces Battalion launched the first phase of their operation by conducting terrain reconnaissance and watching activities on Lima Site 85 to learn their opponent's routines.

As part of the second phase, commenced on 22 January 1968, six PAVN sappers were sent out to climb Phou Pha Thi, to pinpoint opposing positions in and around Lima Site 85, as well as routes of withdrawal.

[19] To capture Lima Site 85, the PAVN Special Forces were equipped with three Chinese-made K-54 pistols, 23 AK-47 assault rifles, four 7.62mm carbines and three RPG-7 rocket propelled grenade launchers.

[17] Shortly after the PAVN Special Forces arrived at the assembly point, they moved off to an undisclosed location for two days to test-fire all their weapons, and to ensure their explosives were in working order.

[20][21] Under the cover of the artillery bombardment, the PAVN Special Forces sent a small team up the mountain to defuse mines and quick-fuse grenades, and to establish their routes of infiltration.

[21] At around 20:20, Sullivan gave the US commander at Lima Site 85 the authority to direct air strikes against targets on the lower slopes of the mountain, on the basis that the situation had become critical.

However, officers of the Seventh Air Force contacted the US Embassy in Laos and indicated that evacuation should only occur as the last resort, when the situation on top of Phou Pha Thi was no longer under their control.

[22] About 02:00, a US adviser at the airstrip reported to Secord and CIA officers at Udorn that he heard gunfire on top of Phou Pha Thi, and communication with US technicians at Lima Site 85 was cut off.

[21] Afterwards, Secord briefed US A-1 Skyraider pilots in Thailand on the situation at Lima Site 85, to familiarize them with friendly positions around the facility, so they could cover the evacuation of US personnel and support the Hmong counterattack.

[27] In response, the PAVN Special Forces organized a defense around the TACAN site, and hid their dead and wounded comrades under the large rocks which dotted Phou Pha Thi.

[25] While US fighter-bombers strafed the TACAN site, the Air America helicopter landed on the airstrip and they picked up two CIA officers, one forward air-controller, and five technicians who hid during the firefight.

[25] Just before midday on 11 March, the USAF turned their attention from looking for their missing personnel to that of destroying the captured radar, along with all the documentation and operation information left behind at Lima Site 85.

In the days following the loss of Phou Pha Thi, Sullivan reflected on the disaster at Lima Site 85 and commented that US technicians operating there should have been evacuated on 10 March, when it became amply clear the PAVN were preparing to launch an assault.

[33] The PAVN victory proved to be a significant one, as they had succeeded in knocking out a major asset of the USAF, which had inflicted heavy damage to North Vietnam's limited industrial infrastructure.

During this period, the 1968 Tet Offensive was underway in South Vietnam, the Marine outpost at Khe Sanh Combat Base was under siege, and there existed an unprecedented flow of enemy logistical traffic which had to be interdicted.

Several mountaineer-qualified JPAC specialists scaled down the cliffs to the ledge where they recovered leather boots in four different sizes, five survival vests, and other fragments of material that indicated the presence of at least four Americans.

[42] On September 21, 2010, Chief Master Sergeant Richard Etchberger's Air Force Cross (awarded posthumously in 1968) was upgraded to the Medal of Honor by President Barack Obama at a White House ceremony for his actions during the Battle of Lima Site 85.