Charlie Haden was sitting in his car one night, listening to the news. Vietnam's neighbour Cambodia was being bombed by the US air force on the orders of President Richard Nixon. Haden felt powerless as an individual - but as a musician he was convinced he could register dissent, and maybe make a difference. He rang his friend and musical collaborator, composer Carla Bley, and said: "Let's do an album about the tragedy of what this administration is doing in the world."
The result was the Liberation Music Orchestra, which made one of the most powerful jazz-driven musical statements of the early 1970s with its self-titled album. The band was a volatile, expensive one, and many members were leaders in their own right, making it difficult to keep it on the road. But in the past 35 years, the LMO has returned whenever the rallying call was loud. It re-formed in 1982, when Ronald Reagan invaded El Salvador, to record the album Ballad of the Fallen. It came back in 1989, during George Bush Sr's time, for a rousing We Shall Overcome at the Montreal jazz festival, and to record the album Dreamkeeper.
And tonight the LMO performs at the Edinburgh festival - the first time Haden and Bley have shared a live performance in 20 years. Haden has long felt angry at what he sees as the Republicans' theft of the election four years ago, and the situation in Iraq has brought that anger to the boil.
Haden has said he always believed in "an America worthy of the dreams of Martin Luther King Jr, and the majesty of the Statue of Liberty". A bespectacled, mild-looking man in his 60s, with chronic bronchitis that worsens on tour, he exudes a firm sense of purpose, disagreeing with the suggestion that, in the end, music is just music: "I wouldn't have done this over all these years if I hadn't believed it made a difference. In recent years in America, it's become very difficult for people critical of the government to express their feelings. Providing some kind of focus for that to happen is power, in its way.
"People have often come up to us after Liberation Music Orchestra gigs and said this music has helped give them confidence to say what they really feel. The material we use draws on a long tradition of people doing that, all over the world."
By background and artistic disposition, Haden is no fan of Republican politics. His father had close friends in the Abraham Lincoln Brigade that fought in the Spanish civil war; as a double-bassist, Haden participated in black Texan saxophonist Ornette Coleman's 1950s revolutions in jazz form. These two influences came together when he heard of the Cambodian bombings. His father had a collection of socialist and anarchist songs of the Spanish civil war, music he and Bley had already considered adapting for a jazz project. The pair turned to this in 1969, examining the Spanish songs and creating arrangements for a full jazz band.
The resulting album featured a suite of 1930s anarchist songs vividly interpreted by the eloquent trumpet of Don Cherry and the hot winds of Argentinian Gato Barbieri's tenor sax. Haden also contributed the brooding Song for Che, a powerful double-bass anthem, plus Ornette Coleman's War Orphans. The sleeve showed the personnel lined up against a brick wall, staring uncompromisingly at the camera from under a banner. The album won the Grand Prix Charles Cros in France (a Grammy equivalent), Swing Journal's Gold Disc award in Japan, and critics' accolades everywhere.
The current line-up includes hot young sax prospect Miguel Zenon and Jazz Passengers trombonist Curtis Fowlkes. Haden, though, says he always hears the band as "timeless, in all its various incarnations". Its music has varied widely, too: politics may inform it, yet Haden is too much of a jazz improviser to settle for a repertoire of marching songs or fists-in-the-air music. On 1989's The Montreal Tapes, the orchestra plays We Shall Overcome for over half an hour: not as a cosy piece of linked-arms chanting but as an increasingly free blues in which a pedigree team of improvisers (including trumpeters Tom Harrell and Stanton Davis, saxophonists Ernie Watts and Joe Lovano, and trombonist Ray Anderson) independently jam themselves into ecstatic spontaneous union.
Earlier this year, when Bley was in London for the Barbican's tribute to film composer Nino Rota, she was still at work on the new LMO scores, and worried Haden would find them too oblique. She was absorbed by the sounds of her Looking for America album, in which she created sublimely sinister mirrors to imagery of John Wayne frontiersmen and roadside shacks bearing Day of Judgment warnings. That music has influenced the current repertoire. "I guess I've taken it more as my own project this time," Bley said then. "I hope Charlie doesn't think it's too ironic."
"She was just worrying," Haden says, now the tour is under way. "But there was nothing for her to worry about. I don't ever compare the current orchestra to the first one, because, although the music might be different, the reason for its existence is the same. Then it was Nixon, now it's George W. What they're doing is the same. So what we're doing is the same, too."