Cathey was inspired to dig further and began work on an independent documentary, CIA in Tibet, which is due to be released next year.

While Cathey was gathering a team to make her film, another researcher, Belgian author Birgit van de Wijer, was interviewing Tibet Task Force veterans in Nepal. In 2007, van de Wijer recorded the stories of 48 former freedom fighters living in three settlement camps: Jampaling, Paljorling and Tashi Gaang, near Pokhara. The camps, paid for with money from the exiled Tibetan government and the CIA, were set up in the 70s to house the veterans following the end of the insurgency.

Now in their 60s, 70s and 80s, most of these men had been monks, farmers or nomads, with little or no education, before they took up arms. Many of them speak about having witnessed Chinese soldiers kill their family and friends or destroy their monastery.

In the years following 1950, when the Chinese asserted sovereignty over Tibet but accorded it a certain amount of autonomy, Tibetans found the soldiers of the People's Liberation Army to be polite and friendly. Gonchoe, a 74-year-old resident of Jampaling, tells van de Wijer: 'The first time the Chinese people came into my country, they were helping people and brought whatever we needed.'

However, by the mid-50s, China had begun to force its agrarian reforms onto people in the east of the country.

'They killed high lamas and were against religion, and they caught people who didn't want to be communists,' Gonchoe says.

After the PLA bombed key monasteries in 1956, wealthy Tibetan businessman Andrug Gompo Tashi organised a rebel army and called it Chushi Gangdruk. He was from Litang, in modern day Sichuan province, a Tibetan town well-known for its anti-Chinese sentiment even today.

In 1957, the Dalai Lama's two brothers, who were in exile by then, put the CIA in touch with Gompo Tashi. The US was keen to use the Tibetans to gather intelligence on China. Starting that year, small groups of Tibetan exiles were smuggled out of India - because of the political sensitivity, Delhi was kept in the dark - and sent to secret CIA training camps, first on Saipan, in the western Pacific, and then at Camp Hale, in Colorado, where they were taught basic spying techniques, morse code, radio operations and guerilla warfare. Co-ordinating with Chushi Gangdruk the men were parachuted back into Tibet on a mission codenamed Operation ST-Circus.

In 1959, everything changed. The Dalai Lama fled to India amid fears the Chinese were plotting his assassination. Chushi Gangdruk soldiers escorted the holy leader across the border and, to this day, many people see this as the single greatest achievement of the Tibetan resistance army. The Chinese were furious and stepped up their attacks, forcing Chushi Gangdruk and thousands of refugees to follow their god king into exile in India.

'The CIA did help but the arms we were getting during that time were not sufficient to fight the Chinese,' says Ratu Ngawang, Gompo Tashi's right-hand man, in footage recorded for Cathey's documentary and made available for this article. 'They were attacking us in brigades and all we had were old guns. They had tanks and we were running on horses. If during that time the CIA had sent the right arms and ammunition to fight, I think that would have really helped us, because during that time all the Tibetans were fearless.'

Fighters reassembled, with thousands of recruits, in Mustang, just across the Tibetan border in northern Nepal. Mustang, at the time a semi-autonomous kingdom, was a good launch pad for raids into Tibet because it has relatively easy-to-traverse mountain passes across the border.

At first, conditions in Mustang were extremely tough for the Tibetan fighters. There was little food. The veterans talk about having to eat boiled yak leather. It wasn't until March 1961 that the CIA made its first airdrop of supplies and weapons. The men would camp for months along the border, drawing maps, tracking movements of Chinese soldiers and occasionally shooting at troops from the hills before fleeing back across the border.

Their main target was the Lhasa-Xinjiang highway. The guerrillas attacked so many army trucks on this route that by 1964 the Chinese had stopped using that stretch, according to van de Wijer.

'We were always doing guerrilla fighting and we often went into Tibet and killed Chinese soldiers,' says Gonchoe. 'We hid a bomb under the road, covered it and when a [Chinese] truck drove over it, it exploded. I did this three times and our group also downed an airplane.'

Tsering, 79, a veteran in Paljorling camp, tells van de Wijer: 'I went a couple of times to Tibet to bomb the road. One time, we attacked a truck and killed 11 soldiers and took all their weapons.'

The CIA continued training Tibetans at Camp Hale, where Cathey's father was an instructor, and parachuting them into Mustang - and Tibet itself, to encourage local resistance groups and to collect more detailed intelligence on the Chinese.

Infiltrations into Tibet were mostly unsuccessful and many of the men sent in by the CIA were killed. Some local Tibetans were suspicious of the US-trained men while others were worried the Chinese would punish them for collaborating. Meanwhile, the Chinese troops were well equipped and had many informers among the villagers.

Bhusang, who was interviewed by Cathey a few months before his death in India last year, was one of seven CIA-trained guerillas dropped into eastern Tibet in March 1961.

'The locals thought we were Chinese spies and they wouldn't give us any food or sell us anything and then we heard that the Chinese knew we were there,' he tells Cathey.

The group fled, pursued by troops to Markham (Mangkang in Chinese), where they joined other fighters and made a stand.

'The Chinese had surrounded us,' he says. 'Thousands and thousands; too many to count, like herds of cattle.'

Bhusang was the only survivor of his seven-man team. A PLA soldier hit him on the back of the head just before he got a chance to bite down on the cyanide pill he had placed in his mouth. He was to spend the next 171/2 years in a Chinese jail.

At Camp Hale, the Tibetan trainees were given Western names - Walt, Tom, Nathan. Bhusang was known as Ken. By all accounts, the men hit it off with their instructors, despite having to communicate through translators. One of the most authoritative published accounts of US involvement, The CIA's Secret War in Tibet, by Kenneth Conboy and James Morrison (a new edition will be published next month), relates how the Americans remember their students.

'[The Tibetans] really enjoyed blowing things up during demolition class but when they caught a fly in their mess hall, they would hold it in their cupped palms and let it loose outside,' says Ray Starke, whose speciality was radio technology.

The Tibetans loved to tease their American teachers. 'They would intentionally leave doors open to get a rise out of me,' says Starke. 'I told them that when I visited them in a free Tibet, I was going to rip their tent flaps off. They thought this was hysterical.'

But things weren't going so well in Mustang. In 1964, Gompo Tashi died from old shrapnel wounds in an Indian hospital. Cross-border raids had become sporadic and generally unsuccessful.

'We got miles of tapes,' CIA officer Angus Thuermer tells Conboy and Morrison, 'but much of it was useless, like Chinese talking about their families back home.'

Internal politics destabilised the resistance and its equipment and military capabilities were lacking. In Mustang, the men would train with sticks because they didn't have enough guns.

In 1967, the CIA began gradually winding down the operation. In the summer of 1974, Nepal, keen to forge stronger ties with China, dispatched its army to shut down the Mustang base altogether. The Tibetans were ready to fight the Nepalese soldiers rather than surrender but, at the 11-hour, the Dalai Lama sent a taped message pleading with them to lay down their arms. A few committed suicide rather than surrender.

Veterans remember that day with heavy hearts.

'I was really very angry and I cried,' says Pema Lhasung in van de Wijer's book. 'I felt pity for myself because I couldn't take revenge for the murder [by Chinese soldiers] of my parents.'

After brief spells in a Nepalese jail, many of the Tibetan veterans settled in the camps near Pokhara or in Kathmandu and worked in businesses that had been set up with money from the CIA: carpet factories, handicraft workshops and a budget hotel in Pokhara, as well as a taxi firm in the Nepalese capital. In the 90s, the carpet businesses went bust.

Over time, many of the freedom fighters left for India. A handful found homes in the West.

'While some of the veterans lead comfortable lives, most are poor,' says Carole McGranahan, an associate professor of cultural anthropology and Tibetan studies at the University of Colorado. 'Some are what I call comfortably poor, with enough food to eat and a roof over their head. But others are not doing so well and rely on the generosity of friends and neighbours, especially those who are single and without a family. The Jampaling camp in Pokhara is known for being especially poor.'

Some of the veterans collect a small pension (US$25 a month) from the Lodrik Welfare Fund, a charity set up to look after the former fighters.

McGranahan has been researching the veteran community and the history of the resistance army since 1994. She has lived with veteran families for extended periods in Pokhara, Kathmandu and three sites in India.

Initially, her research focused on the Kham region, the area now covered by the eastern part of the Tibet Autonomous Region and Tibetan areas of Yunnan and Sichuan provinces. However, McGranahan became intrigued by the same puzzle that struck Cathey: why is this key part of modern Tibetan history so little known?

In Arrested Histories: Tibet, the CIA and Memories of a Forgotten War, which came out last year, McGranahan concludes that there is some truth to the theory that the resistance has been sidelined because it is a story of war and the Dalai Lama's legitimacy rests on his policy of non-violence. But the key reason, she writes, is that the resistance army challenged the status quo of aristocratic Tibetan society, in which wealthy landowners and the Buddhist clergy ruled from Lhasa. The resistance army was made up of men who had, until then, never wielded any power over the fate of the nation.

Since the late 90s, McGranahan says, a focus on democracy by the Dalai Lama has meant the secret war is experiencing a 'period of release'. A moment the veterans have long desired.

'I always tell my own children ... if somebody asks you who your father is, you can be very proud because I made history,' Ahnzin, a 68-year-old veteran at Jampaling, tells van de Wijer. 'Never forget what I did.'

As part of this period of release, Lhamo Tsering (a resistance fighter who helped set up the Mustang camp with Gompo Tashi) has written a 15-volume account of the resistance. Chushi Gangdruk commander Ratu Ngawang has also recently written a book on the war. Both books have been published in Tibetan.

In 1998, Lhamo Tsering's son, Sonam Tsering, made The Shadow Circus: The CIA in Tibet, a BBC-commissioned documentary on the secret war. Says McGranahan: 'I find it interesting that soon we will have two films about the resistance: one by the child of a Tibetan leader and one by the child of an American CIA officer. To me, this is further evidence of how even in the families of men who were directly involved, the story of the resistance was not fully known, or, in Lisa's case, even peripherally known.'

'[The] Dalai Lama told me to put all my experiences into words and specifically asked me not to exaggerate or not to lie in any circumstances,' Ratu Ngawang tells Cathey.

Not so on the Chinese side.

'The CIA makes for a convenient bogeyman [for the Chinese] ... Discontent in Tibet can be rather neatly blamed on outside foreign agitation,' says Jeremiah Jenne, a history researcher at the University of California, Davis.

'Veterans [have accepted the suppression of their history] because they see it as coming from the Dalai Lama,' says McGranahan. 'Betrayed is not the emotion these men feel. Frustration, yes, but betrayal, no. The reason why is because of their overwhelming devotion to the Dalai Lama as religious leader as well as political leader.'

This loyalty explains why the veterans did not feel betrayed when, in the late 80s, the Tibetan government in exile and the Dalai Lama softened their demands to autonomy for Tibet instead of independence, the so-called Middle Way.

'They all support the Dalai Lama's stance,' Cathey says. 'Their generation is still extremely loyal to him and wouldn't go against his views, regardless of their personal feelings.'

One veteran who breaks the mould is US-based author and activist Jamyang Norbu. Born in India, Norbu ran away from home when he was a teenager to join the Mustang forces. He arrived quite late in the game, in 1971, and thus missed out on the fighting.

'I never got to shoot anybody,' the 61-year-old laughs. 'It's my greatest regret.'

He was in Japan when he heard the news about the policy change.

'I got a call from the Dalai Lama's older brother [Thubten Jigme Norbu]. He was almost in tears. He said: 'They've sold our independence.' I felt His Holiness and the people around him had been very naive. I thought it was ridiculous. They have no idea about the Communist Party of China. I felt really disappointed. I wouldn't say betrayed, I would say completely disappointed.'

All the veterans in van de Wijer's book speak lovingly of their religious leader - 'I remain happy by praying to the Dalai Lama. He is great,' notes 80-year-old Jampa Norbu - but some retain a fighting spirit.

Jampa Choedak, an 80-year-old monk, tells van de Wijer: 'I have been fighting the Chinese and now I am old and am still willing to fight to get my country back. But there are some people who are speaking of the Middle Way, or another way, but I don't like these options, I just want to fight the Chinese to get my country back.'

One of the tragic aspects of the secret war was the belief among the guerillas that the US genuinely wanted to help them regain independence, even when the assistance given was trifling.

'It wasn't even one thousandth of the help we needed,' says Jamyang Norbu. 'Tibetans by and large believed that the US would support the whole Tibetan cause, primarily because of America's reputation at that time,' he says. 'The US wasn't just another colonial power like France or Britain ... it had liberated China and so on. We felt it was a wonderful thing, this help.'

CIA documents relating to the Tibet Task Force are still classified. However, former CIA official Sam Halpern told The Shadow Circus documentary makers it was only ever intended as a 'nuisance' mission.

'I think, basically, the whole idea was to keep the Chinese occupied somehow. Nobody wanted to go to war over Tibet,' he said.

'For the majority of Tibetans with whom I worked, discomfort with American empire building on the back of the Tibetan struggle pales in comparison to the gratitude for their support,' says McGranahan.

While the CIA did not help the Tibetans win back their country, it did, say some, help them lay the foundations for a country in exile. The CIA air dropped weapons and supplies just before the Dalai Lama fled Lhasa and that helped the Chushi Gangdruk forces escort the spiritual leader and thousands of refugees across the border, notes Jamyang Norbu.

'There would never have been any Tibetan refugees because [China] would have sealed off the border. But because the guerrillas maintained this road in 1959, we had cut off the whole Chinese garrison on the way south to India.'

The portraits of the men in van de Wijer's book are mostly sorrowful; their eyes tired and rheumy; hair cropped short, furrowed faces sunken with age. Totalling more than 10,000 in the 1960s, the surviving veterans now number only in the hundreds.

Bhusang explains in the recording made just before his death what kept him going.

'All these years I've been treated like an animal,' he says. 'I've been tortured on and on and still I've been able to survive. Whenever the Chinese tortured me, I thought: 'Such people exist in the world and we have done nothing to them.' I pray to the Dalai Lama and am proud of it - proud I have been able to survive and to have done my bit when I was in my own country.'

The interviews with the CIA-trained veterans and their portraits have been published in Birgit van de Wijer's book Tibet's Forgotten Heroes: The Story of Tibet's Armed Resistance Against China.