The Indochina War 50 years on: A forgotten battle atop Laos's sacred mountain

Shrouded in morning mist, the Phou Pha Thi massif looms large, an ominous, solitary colossus, astride the jagged limestone peaks of northeast Laos that peer over the nearby border into Vietnam.

For the people of the mountains, the Lao Hmong, Phou Pha Thi is 'Sacred Mountain' but in the so called 'Secret War' of the 1960s and 70s waged by the Central Intelligence Agency (CIA) and its proxies, it bore a military moniker, Lima Site 85.

Pha Thi is a natural mountain fortress and during successive Vietnam wars served as a key military observation post overlooking the nearby border – qualities quickly appreciated by French colonial forces, their CIA successors and Hmong guerilla allies.

But the mountain's transformation into a Top Secret radar guidance site for bombing missions would end in a surprise assault in 1968 by North Vietnamese special forces sappers, the deaths of 12 US Air Force technicians manning the facility and, to this day continue to raise a host of unanswered questions.

“The Americans were defeated here,” said my guide, a young junior soldier based at the small Lao army outpost at the foot of the mountain.

As the end of the Second Indochina War is remembered on its 50th anniversary with commemorations of communist victories in Vietnam – more subdued in Cambodia because it also marked the start of the Khmer Rouge “Killing Fields” reign of terror – it is timely to reassess events at Lima Site 85 and a consequential battle fought in a remote corner of northeast Laos.

But first some context. The 1962 Geneva agreements signed by the Cold War protagonists banned the presence of foreign troops in Laos – measures designed to ensure Laotian neutrality. This was ignored by both sides.

From 1959 two proxy forces engaged in an escalating civil war – the communist Pathet Lao, backed by Hanoi and the Soviet Eastern Bloc, and the Royal Lao Army, supported by the US and Thai mercenary allies. As the war see-sawed into the 1960s a swathe of northeast Laos was occupied by thousands of North Vietnamese troops. 

In response the US, through the CIA, funnelled covert arms to the Royal Lao Government and anti-Vietnamese Hmong guerillas, setting the stage for a major escalation of hostilities in a conflict that became known as the Secret War.

In August 1966, as the Vietnam War intensified so did US Air Force (USAF) bombing of the North, targeting Hanoi, Haiphong and suspected enemy border sanctuaries. Weather was a critical factor in the bombing campaign with the onset of the northeast monsoons from October to April severely hindering air operations. A radar guidance system allowing a straight shot towards Hanoi could fix the navigational problem.

Under the codename Project Heavy Green, the USAF with CIA assistance, built a tactical air navigation station and landing pad for helicopters on the Pha Thi summit. A TACAN navigation beacon had been installed earlier. At the base was a short dirt landing strip for resupply flights.

Neutrality concerns meant the mountain top site was staffed by 'sheep dipped' USAF personnel posing as civilian employees of the Lockheed company, sworn to total secrecy.

The new radar, an MSQ-77, would guide aircraft to a precise point where their bomb load could be released regardless of cloud cover or weather and was regarded as accurate enough for targets such as airfields and industrial sites.

But Lima Site 85's location in enemy heartland, just 24 kms from the Vietnamese border and 48 kms from the main Pathet Lao stronghold of Xam Neua presented serious security concerns. US Ambassador William Sullivan in Vientiane was especially apprehensive but ruled out deploying any US troops to defend the installation.

Instead, security would be entrusted to a battalion of Hmong guerillas 200 of whom would be deployed in the immediate vicinity of the radar site bolstered by a handful of Thai mercenaries.

The increased activity around Lima Site 85 attracted attention. Pathet Lao forces were active in the area, and clashes erupted with the Hmong defenders, prompting additional protective measures, including land mines and concertina wire around the facility's sole accessible approach. The sheer north and eastern sides of Lima Site 85 where the cliff faces rose to between 85 and 90 degrees, were regarded as virtually impossible to ascend.

Weather conditions across North Vietnam were so severe between November 1967 and March 1968 that the USAF relied on Lima Site 85 for 23 percent of all strikes on northern Vietnam.

A response was not long in coming, although to the defenders it initially might have seemed comical. On January 12, two Soviet-made AN-2 'Colt' biplanes of the North Vietnamese Air Force attempted to bomb the summit, dropping modified 120mm mortar rounds from the aircraft. 

A 'Huey' helicopter operated by the CIA gave chase. It caught up with the lumbering biplanes, and a flight mechanic, firing a sub machine gun from the open door, managed to hit both aircraft, downing one, which crashed and burnt. The second plane was also damaged and crashed soon after.

But the military situation worsened. By February, the mountain was surrounded by enemy forces. Still, the USAF resisted calls to abandon the base, citing its critical role in ensuring accurate targeting for bombing operations and in the face of fresh warnings from Sullivan and the CIA that base security could no longer be guaranteed.

On March 11, the last day of radar operations, 19 Americans were on the mountain top, 16 Heavy Green personnel, a combat controller, and two CIA paramilitary officers in their own building located a short distance away near the helipad.

Some 3000 enemy soldiers were positioned around the mountain which they began to shell throughout March 10, forcing the Heavy Green technicians into a shelter on a rock ledge below the peak, accessed by a cargo net dropped over the cliff face.

Around midnight, 33 North Vietnamese sappers armed with grenades, rocket launchers and assault weapons, who had trained for several months on similar peaks, began the perilous climb up the eastern side of the mountain, a feat deemed impossible by US security officials.

They emerged close to the summit near the radar facility and attacked in the early hours of the morning targeting accommodation buildings, offices and radar hut resulting in the first American casualties. Then they discovered the American's hiding place, throwing grenades and firing automatic weapons. Several US personnel were killed outright and others wounded in this prolonged exchange in which the attackers were kept at bay by return fire from a sole air force sergeant armed with an M16.

Back in Vientiane, Ambassador Sullivan ordered an immediate evacuation of the site which began at 7.15 am utilising USAF and Air America helicopters, while fierce fighting continued around the base of the mountain and clashes across the peak as the NVA force sought to secure the mountain top.

Seven Americans were rescued but 12 USAF personnel were killed – the biggest single ground combat loss of the USAF during the Vietnam War. The bodies of 11 servicemen killed in the fighting were left on the summit where they fell, a cause of considerable controversy when details of the attack were eventually trickled out.

With the summit now in North Vietnamese hands, the mountain top was first napalmed then bombed to prevent sensitive equipment being seized, a decision taken after assessing the 11 missing were likely already dead.

Then in March, President Lyndon Johnson announced a partial halt to bombing over the north. Phou Pha Thi was never reoccupied, and the story of Lima Site 85 remained classified under a veil of official silence.

In 1977, Lima Site 85 received a brief mention in the official USAF history of the Vietnam War but only a passing reference to its use as an air navigation aid and nothing of its capture or true targeting mission.

An official Vietnamese war history report titled “Raid on the TACAN Site atop Pha-Thi Mountain by Military Region Sapper Team on 11 March 1968” was published in 1996. It contained one important revelation – that no prisoners were taken during the attack, partially allaying fears and unsubstantiated claims of missing, unreturned POWs.

With Lao-US bilateral relations improving by the 1990s, work began to resolve the fate of the missing  servicemen with a visit to the site in 1994 by a team from the Joint POW/MIA Accounting Command. It would take until 2003 for the first breakthrough when two former Vietnamese commandos who had taken part in the attack revealed they had thrown several American bodies over the cliff. A follow-up search by US mountaineering search and rescue specialists did find evidence of human remains and military equipment resulting in the identification of one of the 11 missing airmen. More cliff face excavations are planned.

Today, Phou Pha Thi is not listed as an official Lao tourist attraction, although it is accessible to visitors. The site falls within a military zone, but I was able to find a young soldier to act as guide and willing to show me the old battleground last month.

A series of dizzyingly steep steel stairs have replaced the old wartime rungs fastened to the boulders and cliffs leading to the summit.  Near the top, we passed a sign warning of unexploded munitions and landmines, a reminder of the terrible wartime legacy bequeathed to Laos – per capita, the most heavily bombed country in the world.

In this anniversary year, there is an irony that Dien Bien Phu lying close by across the border should have served as a warning to Americans about the vulnerability of operating remote military facilities – no matter how vital - in enemy heartland, or the tragic consequences of prioritising strategic objectives over the safety of its personnel.

But Phou Pha Thi serves another painful reminder of the deadly consequences of the role it played in the American air war – and the toll on Lao civilians.

It’s a sobering fact that since the end of the Vietnam War, more than 20,000 Laotians have been killed or injured by unexploded ordnance mostly from US bombing, from the 2 million tonnes dropped up until 1973 on this impoverished Southeast Asian country of six million people. Five decades after the war, the deadly legacy of unexploded munitions and bombs remains for many, a daily reality.

Mark Dodd is a Walkley Award journalist and former Reuters Cambodia bureau chief (1991-95). He served as a foreign correspondent for 12 years and was defence/foreign affairs writer for The Australian (2006-2012). Dodd holds masters degrees in Special Operations and Irregular Warfare and International Relations.