While America reverberates with self-righteous rhetoric about how it is bringing freedom to Iraq, its own indigenous peoples are far from liberated. A powerful exhibition of photographs taken among the Crow tribe on America's largest native reservation highlights this glaring irony.

Alberto Arzoz is a Basque photographer based in London. Several of his portraits hang in the National Portrait Gallery, and he has contributed pictures to the Guardian, the Telegraph and the Times. His fascination with Native American culture began at the start of his career. On a trip to New York to promote Basque culture, he saw a Native American, wearing tribal dress and busking on a street corner. "I am a Sioux chief," the man said. "Please give me money. I need money to go back to my reservation." That passing encounter awoke an interest in the young photographer that has endured 15 years.

As a Basque, Arzoz sensed a particular affinity with the oppression of Native Americans. "I felt it in my own culture - not being able to speak the language, not being able to have your own folklore, not being able to be yourself." Over the next decade, he read every book he could find on the subject, and in 2002 he returned to America, determined to visit a reservation. At a Fourth of July cultural display in Central Park, he met a group of Native Americans who invited him to the Crow Fair, the biggest Native American festival in the US, attended by representatives of every tribe in the country. "They call it the tepee capital of the world."

Arzoz found the Crow Fair fascinating, but he wanted to go deeper. He realised that although he had spent his whole life surrounded by "Red Indian" clichés, from the westerns of his childhood to today's New Age knick-knacks, he had no true understanding of the real thing. The following year he borrowed £5,000 to cover his expenses and equipment, and travelled to the Crow reservation in Montana. He found a unique culture that had endured despite generations of social and economic strangulation. The result was the photos now on show for the first time at the Horniman Museum in London.

That the Crow tribe has survived at all is a miracle. The persecution of the Crows by white invaders culminated in 1840, when the US army gave them blankets infected with smallpox - a wicked masquerade of kindness. In 1851 they were confined to a reservation of about 40 million acres, which subsequently shrank to less than one-tenth of its original size. About a third of this land is now leased to (and run by) white Americans.

Christian missionaries have eroded the Native American religion, and white-run businesses find easy pickings in a place where labour is cheap and locals find it very hard to get loans to start businesses of their own. Many of the 12,000 Crows in the US are on food stamps, and there are problems with drugs and alcohol. "The American dream is not for them - I call it the American Nightmare," says Arzoz. "Native Americans cannot participate in mainstream American life. Some tribes have been relocated to swamps, to deserts." At least the Crows still cling to a slivver of their original land.

Although Arzoz describes the situation as "heartbreaking", he found many things on the reservation that inspired him. His photograph of a sweat lodge shows the Crows' most sacred ritual: a religious ceremony in a darkened tepee filled with steam. The tepee is the womb, the darkness is human ignorance, and the glowing stones are the origins of life.

Arzoz also photographed the ceremonial slaughter of a bison, part of a small protected herd that survived the carnage by white hunters which destroyed the Crows' livelihood and way of life. "For them, the bison was not only an animal. It was the embodiment of God."

Also on the Crow reservation is the site of the blood-soaked defeat of Lieutenant General George Custer at the Battle of Little Big Horn in 1876. There has been a memorial to the white fatal- ities there since 1881. In 1991, its name was changed from the Custer Battlefield National Monument to Little Bighorn Battlefield National Monument. A Native American "Peace Through Unity" memorial was finally erected alongside it to honour the Native Americans who also died during Custer's last stand.

These small changes are a ray of hope, but Arzoz still feels pessimistic. "What the extermination of the bison couldn't do, TV is doing," he says. "In the end, that will do more damage." And the process is all one-way. None of Arzoz's white American friends - liberal New Yorkers - knew anything about Native Americans. He believes that the only future for these tribes is to make cultural connections beyond the United States.

"They are very laid-back people," Arzoz says. "They don't have that competitive edge. That doesn't help them in terms of fighting back."

Susan Stewart Medicine Horse is a curator at the Chief Plenty Coups Museum and State Park on the Crow reservation, and her husband is a Sundance chief. She is more optimistic than Arzoz. "Compared to 20 years ago, I think that things have improved somewhat," she says. "We have a little bit more self-determination."

Yet, as her tribe tries to pursue a double life, the age-old dilemmas remain. How do you straddle two worlds? How do you become bicultural - and yet not lose your sense of identity? This exhibition asks the same questions. The answers will remain as elusive as they have ever been until other people learn to care about America's original, indigenous culture.

The Crow: 21st-century Native Americans is at the Horniman Museum, London SE23 (020 8699 1872) from 6 May until 15 October. For further information, visit www.horniman.ac.uk

Native America: a culture on the brink

There are 4.1 million Native Americans, belonging to 563 federally recognised tribes, currently living in the US.
The largest tribes are the Cherokee, Navajo, Choctaw, Sioux, Chippewa, Apache, Lumbee, Blackfoot, Iroquois and Pueblo.
The average life expectancy of a Native American is 65 years, compared to 73.3 years for non-Native Americans.
Unemployment among Native Americans in the United States stands at 46 per cent. On some reservations, it is as high as 65 per cent.
Ninety thousand Native Americans are homeless or under-housed.
14.7 per cent of Native American homes are overcrowded, compared to 5.7 per cent of homes among the general population in the US.
Rates of drug addiction among Native Americans over the age of 12 are higher than for any other American group.

Research by Ally Carnwath