The Hmong hill tribe in Laos was recruited in 1961 by the CIA, to fight a "secret war" against the Communists of North Vietnam. Their job was to try to block the Vietcong's supply route.
Known as the "Ho Chi Minh trail", it ran through Laos, along the border with Vietnam. More than 40,000 Hmong were killed in the fighting that followed.
When the US fled Saigon in 1975, Communists also seized control of Laos.
The Hmong, abandoned by the US, allegedly became the target of retaliation and persecution.
This marked the beginning of the mass exodus of Hmong refugees into Thailand, which eventually swelled to more than 300,000.
Some of them were eventually resettled in the US, but many continue to languish in Thai detention camps. And at least 7,000 Hmong are still hiding in the mountains and jungles of Laos, including some former CIA fighters.
Under constant threat of attack by the army - they are a lost tribe - forgotten by the world for more than three decades.
Our correspondent Tony Birtley trekked from the town of Phonsavan to reach their jungle hideout - the first television journalist ever to do so.
Watch part one of this episode of The Lost Tribe on YouTube
Watch part two of this episode of The Lost Tribe on YouTube
The Lost Tribe will broadcast at the following times GMT:
Thursday, March 13 07:30, 19:30 Friday, March 14 16:30 Saturday, March 15 13:30 Sunday, March 16 03:00, 20:30 Monday, March 17 05:30 Tuesday, March 18 14:30 Wednesday, March 19 01:30, 11:30
To contact us or leave a comment click on 'Send your feedback' at the top of the page.
Watch Al Jazeera English programmes on YouTube
Join our debates at the "Your Views" section of the site.