'If you start looking at them as humans, then how are you gonna kill them?'

They are a publicity nightmare for the US military: an ever-growing number of veterans of the Iraq conflict who are campaigning against the war. To mark the third anniversary of the invasion this month, a group of them marched on Katrina-ravaged New Orleans. Inigo Gilmore and Teresa Smith joined them

At a press conference in a cavernous Alabama warehouse, banners and posters are rolled out: "Abandon Iraq, not the Gulf coast!" A tall, white soldier steps forward in desert fatigues. "I was in Iraq when Katrina happened and I watched US citizens being washed ashore in New Orleans," he says. "War is oppression: we could be setting up hospitals right here. America is war-addicted. America is neglecting its poor."

A black reporter from a Fox TV news affiliate, visibly stunned, whispers: "Wow! That guy's pretty opinionated." Clearly such talk, even three years after the Iraq invasion, is still rare. This, after all, is the Deep South and this soldier less than a year ago was proudly serving his nation in Iraq.

The soldier was engaged in no ordinary protest. Over five days earlier this month, around 200 veterans, military families and survivors of hurricane Katrina walked 130 miles from Mobile, Alabama, to New Orleans to mark the third anniversary of the Iraq war. At its vanguard, Iraq Veterans Against the War, a group formed less than two years ago, whose very name has aroused intense hostility at the highest levels of the US military.

Mobile is a grand old southern naval town, clinging to the Gulf Coast. The stars and stripes flutter from almost every balcony as the soldiers parade through the town, surprising onlookers. As they begin their soon-to-be-familiar chants - "Bush lied, many died!" - some shout "traitor", or hurl less polite terms of abuse. Elsewhere, a black man salutes as a blonde, middle-aged woman, emerging from a supermarket car park, cries out, "Take it all the way to the White House!" and offers the peace sign.

Michael Blake is at the front of the march. The 22-year-old from New York state is not quite sure how he ended up in the military; the child of "a feminist mom and hippy dad", he says he signed up thinking that he would have an adventure, never imagining that he would find himself in Iraq. He served from April 2003 to March 2004, some of that time as a Humvee driver. Deeply disturbed by his experience in Iraq, he filed for conscientious objector status and has been campaigning against the war ever since.

He claims that US soldiers such as him were told little about Iraq, Iraqis or Islam before serving there; other than a book of Arabic phrases, "the message was always: 'Islam is evil' and 'They hate us.' Most of the guys I was with believed it."

Blake says that the turning point for him came one day when his unit spent eight hours guarding a group of Iraqi women and children whose men were being questioned. He recalls: "The men were taken away and the women were screaming and crying, and I just remember thinking: this was exactly what Saddam used to do - and now we're doing it."

Becoming a peace activist, he says, has been a "cleansing" experience. "I'll never be normal again. I'll always have a sense of guilt." He tells us that he witnessed civilian Iraqis being killed indiscriminately. It would not be the most startling admission by the soldiers on the march.

"When IEDs [Improvised Explosive Devices] would go off by the side of the road, the instructions were - or the practice was - to basically shoot up the landscape, anything that moved. And that kind of thing would happen a lot." So innocent people were killed? "It happened, yes." (He says he did not carry out any such killings himself.)

Blake, an activist with IVAW for the past 12 months, is angry that American people seem so untouched by the war, by the grim abuses committed by American soldiers. "The American media doesn't cover it and they don't care. The American people aren't seeing the real war - what's really happening there."

We are in a Mexican diner in Mississippi when Alan Shackleton, a quiet 24-year-old from Iowa, stuns the table into silence with a story of his own. He details how he and his comrades in Iraq suffered multiple casualties, including a close friend who died of his injuries. Then he pauses for a moment, swallows hard and says: "And I ran over a little kid and killed him ... and that's about it." He has been suffering from severe insomnia, but later he tells us that he has only been able to see a counsellor once every six weeks and has been prescribed sleeping pills.

"We are very, very sorry for what we did to the Iraqi people," he says the next day, holding a handwritten poster declaring: "Thou shall not kill."

As we get closer to New Orleans, the coastline becomes increasingly ravaged. Joe Hatcher, always sporting a keffiyeh and punk chains, reflects on his own time in the military and the hostility he has met from pro-war activists at home in Colorado Springs, Colorado, a town with five army bases where he campaigns against the war at town hall forums. He says: "There's this old guy, George, an ex-colonel. He shows up and talks shit on everybody for being anti-war because 'it's ruining the morale of the soldier and encouraging the enemy'.

"I scraped dead bodies off the pavements with a shovel and threw them in trash bags and left them there on the side of the road. And I really don't think the anti-war movement is what is infuriating people."

When we reach Biloxi, Mississippi, the police say that there is no permit for the march and everyone will have to walk on the pavement. This is tricky because Katrina has left this coastal road looking like a bomb site.

Jody Casey left the army five days ago and came straight to join the vets. The 29-year-old is no pacifist; he still firmly backs the military but says that he is speaking out in the hope of correcting many of the mistakes being made. He served as a scout sniper for a year until last February, based, like Blake, in the Sunni triangle.

He clearly feels a little ill at ease with some of the protesters' rhetoric, but eventually agrees to talk to us. He says that the turning point for him came after he returned from Iraq and watched videos that he and other soldiers in his unit shot while out on raids, including hour after hour of Iraqi soldiers beating up Iraqi civilians. While reviewing them back home he decided "it was not right".

What upset him the most about Iraq? "The total disregard for human life," he says, matter of factly. "I mean, you do what you do at the time because you feel like you need to. But then to watch it get kind of covered up, shoved under a rug ... 'Oh, that did not happen'."

What kind of abuses did he witness? "Well, I mean, I have seen innocent people being killed. IEDs go off and [you] just zap any farmer that is close to you. You know, those people were out there trying to make a living, but on the other hand, you get hit by four or five of those IEDs and you get pretty tired of that, too."

Casey told us how, from the top down, there was little regard for the Iraqis, who were routinely called "hajjis", the Iraq equivalent of "gook". "They basically jam into your head: 'This is hajji! This is hajji!' You totally take the human being out of it and make them into a video game."

It was a way of dehumanising the Iraqis? "I mean, yeah - if you start looking at them as humans, and stuff like that, then how are you going to kill them?"

He says that soldiers who served in his area before his unit's arrival recommended them to keep spades on their vehicles so that if they killed innocent Iraqis, they could throw a spade off them to give the appearance that the dead Iraqi was digging a hole for a roadside bomb.

Casey says he didn't participate in any such killings himself, but claims the pervasive atmosphere was that "you could basically kill whoever you wanted - it was that easy. You did not even have to get off and dig a hole or anything. All you had to do was have some kind of picture. You're driving down the road at three in the morning. There's a guy on the side of the road, you shoot him ... you throw a shovel off."

The IVAW, says Hatcher, "is becoming our religion, our fight - as in any religion we've confessed our wrongs, and now it's time to atone."

Just outside New Orleans, the sudden appearance of a reporter from al-Jazeera's Washington office electrifies the former soldiers. It is a chance for the vets to turn confessional and the reporter is deluged with young former soldiers keen to be interviewed. "We want the Iraqi people to know that we stand with them," says Blake, "and that we're sorry, so sorry. That's why it was so important for us to appear on al-Jazeera."

A number of Vietnam veterans also on the march are a welcome presence. For all the attempts to deny a link between the two conflicts, for both sets of veterans the parallels are persuasive. Thomas Brinson survived the Tet offensive in Vietnam in 1968. "Iraq is just Arabic for Vietnam, like the poster says - the same horror, the same tears," he says.

Sitting on a riverbed outside New Orleans, Blake turns reflective. "I met an Iraqi at one of the public meetings I was talking at recently. He came up to me and told me he was originally from the town where I had been stationed. And I just went up to this complete stranger and hugged him and I said, 'I'm sorry. I'm so sorry.' And you know what? He told me it was OK. And it was beautiful ..." He starts to cry. "That was redemption".