Mejia, 30, never did go back. He went on the run for five months, staying with friends and relatives, using only cash, travelling by bus and not calling his mother or daughter, before he turned himself in as a conscientious objector. A military tribunal sentenced him to one year in prison.

Like Mejia, 24-year-old Darrell Anderson went on the run just a few days before he was due to redeploy. "I was supposed to leave for Iraq on January 8th. On the 3rd I started to talk to people about the war. By the 6th I woke up and had hit a brick wall. I just knew I wasn't going to be able to live a normal life if I went back."

He told his mother, Anita, who said she "had been hoping for that". "I packed up the car and took him to Canada. It was the first time I slept through the night in two years," she says. Anderson is now essentially a fugitive seeking asylum in Canada.

And then there was Joshua Casteel, an interrogator at the notorious Abu Ghraib prison. His turning point came when a 22-year-old Saudi who came to Iraq for jihad was brought before him for questioning. "He admitted it," says Casteel, 26, a deeply religious Catholic convert from Cedar Rapids, Iowa. "I asked him why he had come to Iraq to kill. Then he asked me why I had come to Iraq to kill. He said I wasn't following the teachings of Jesus, which was pretty ironic. But I thought he sounded just like me. He was not a maniacal kind of killer. He had never fired a weapon in his life ... I know what it's like to proselytise. At one time I had been a pretty nationalistic kid. I understood where he was coming from but in order to do my job I couldn't look at him as a human being. I had to look at him as an object of exploitation."

Two days later Casteel went to Qatar on leave. When he came back he told his commander that he would be applying for conscientious objector status. "I said I wouldn't turn in my weapon while I was there or talk to the media but would carry on doing my job and when I got back home I would ask to leave the military." He filed his application on February 16 and was granted an honourable discharge on May 31.

Whether you call them deserters, conscientious objectors or resisters, every story of American soldiers who left the army prematurely because of the Iraq war shares the same emotional trajectory. They begin with doubt and end with determination. And somewhere along the way comes that ill-defined but crucial moment when the psychological struggle and moral angst overwhelm their military commitment.

The number applying for conscientious objector status has quadrupled since 2000 but remains small, though many more simply go awol. In 2004, 110 soldiers filed, of whom around half were successful. The rest went back to war, refused to serve, were jailed or are still in hiding. Yet there has been a huge increase in enquiries, according to JE McNeil, director of the Centre on Conscience and War. Before 9/11, she says, its GI hotline received roughly one phone call a month from those seeking information about how to get out of the military. In the year after, it went up to one or two a week. Currently it stands at more than one a day.

Which could explain the army's increasingly hardline attitude towards deserters. In the past the overwhelming majority of deserters (94%) were released - if not with an honourable discharge, at least with little fuss. But as the war on terror started, the military had to get tough on those who went missing. Shortly after 9/11 it issued new rules that deserters should be returned to their military units for evaluation. In May 2004, Major General Claude Williams of the Army National Guard issued an internal memo saying: "Effective immediately, I am holding commanders at all levels accountable for controlling manageable losses." He ordered commanders to retain 85% of the soldiers who were scheduled to end their active duty and "execute the awol recovery procedures for every awol soldier".

In one instance, one of those in command had a change of heart. In June Ehren Watada became the first commissioned officer to refuse to deploy. "This war is not only morally wrong but illegal under international and American law," he said. "I took an oath to defend the laws and constitution. My participation would make me party to war crimes." When we spoke, Watada's unit was due to ship out in a matter of days and he was getting ready to do time.

In July he was charged with conduct unbecoming an officer, "missing movement" and contempt toward officials. "I will probably go to jail but I think it's my duty to say it's not a lawful order," says Watada, who plans to challenge the legality of the war at any trial that may ensue. If convicted he could face nearly eight years in prison and a dishonourable discharge.

There are at least 50 ways to leave your regiment. Many simply go absent without leave and hope they are never found, others flee to Canada or apply for release as a conscientious objector. Some pursue less confrontational avenues. "People try for medical discharges, or discharge on grounds of hardship," says McNeil. "They take drugs and hope they get caught. They come out as gay."

A few resort to truly desperate measures. In December 2004, Marquise Roberts, 24, got his cousin, Roland Fuller, to shoot him in the leg , then told the police he'd been struck by a stray bullet. "I was scared," he told police after they found no blood or casings in the area and the cousins couldn't keep their story straight. "I didn't want to go back to Iraq and leave my family. I felt that my chain of command didn't care about the safety of the troops. I just know that I wasn't going to make it back." Fuller was sentenced to up to 30 months in prison; Roberts got a year in military prison; his wife, Donna, who helped them, got four years' probation.

The process for becoming a conscientious objector is both involved and tough. Soldiers have to show that they are opposed to all wars, not just a particular war. They must also inform their commanding officer, who then appoints an investigating officer. The investigating officer arranges for the soldier to be interviewed by both a chaplain and a psychiatrist, both of whom write reports. Then the investigating officer writes a report and, finally, the commanding officer delivers his verdict in his own report. This usually takes between 12 and 18 months, during which time the soldier must remain with his or her unit. "The standard is pretty high and the military can be capricious about following its own standards," says McNeil. "Basically it's a crap shoot. And you're still in the military until they decide. The only thing they can't make you do during that time is pick up a weapon. The response of your colleagues can vary. Some soldiers have been raped; others were told, 'I don't agree with you but I'll support you any way I can.' "

Desertions - those who leave without permission - rose steeply from 1,509 in 1995 to 4,719 in 2001, only to drop again last year to 2,500. For soldiers to be classified as deserters, they must be awol for 30 days. At that point they are dropped from the military rolls and a federal warrant is issued for their arrest, although for many years they were rarely pursued for lack of resources.

Jeffry House, a Toronto-based lawyer fighting through the Canadian courts for political asylum for soldiers escaping the military, says he has seen a "steady trickle" of soldiers make it across the border. House made the same journey himself in 1970 after he came up number 16 on the draft lottery for Vietnam. He has 12 clients and knows of around 25 more being represented by others and another 200 "in other situations" in Canada. His bid to gain the deserters political asylum now sits with the Canadian federal court of appeal, having been rejected by lower courts, but he is convinced they won't get sent back. "I'd be very surprised," he says. "It would run contrary to everything Canadians think about themselves."

These figures do not represent an exodus in terms of the overall size of the US military, which stands at roughly 2.3 million (including reservists), or compared with historical rates of desertion and conscientious objection during other wars. During the Korean war 4,300 soldiers were granted conscientious objector status; during Vietnam between 50,000 and 90,000 came to Canada, mostly as draft dodgers rather than deserters.

None the less, in an army that is overextended and where recruitment is proving increasingly difficult, every soldier counts. More than 2,600 US soldiers have been killed in the Iraq war and around 19,500 have been injured. They have to be replaced. Since 2001 the military has taken extraordinary steps to bolster its depleted ranks. There is currently a push to attract non-citizens to the service and to lift the upper age limit for new recruits. And, over the past few years, the military has raised by half the rate at which it grants "moral waivers" to potential recruits who have committed misdemeanours and lowered the educational level required. Steven Green, the former soldier accused of raping a 14-year-old Iraqi girl and murdering her family in Mahmudiya, entered the military on one such waiver.

Reservists, for whom the military was a part-time commitment, are seeing their tours of duty in Iraq extended and the defence department has once again imposed "stop loss" orders - refusing to allow a military member to leave or retire once their required term of service is complete.

The political rhetoric from the Bush administration and Congress maintains that the nation must stay the course in Iraq. "It's time for this House of Representatives to tell the world that we know our cause is right and that we are proud of it," Republican Speaker of the House Dennis Hastert said during a debate on withdrawal in June. The next day the House rejected setting a timetable by 256 to 153. Hastert and his colleagues, however, have yet to convince the people fighting the war. A Zogby International/Le Moyne College survey earlier this year revealed that 72% of troops said the US should withdraw within 12 months, while 29% said they should pull out immediately.

To grasp fully why some troops go awol, one must look beyond the polls to what made them join in the first place. All have their own reasons but Darrell Anderson's story is the one you hear most often. "I was trying to get into college," he says. "I was living in a trailer with my grandmother. I was broke and I needed education and healthcare, and if I had to go to war for it then that was just what I had to do. Going to the military was my last chance. My last option." He describes the circumstances that shaped his choice with a resigned smile and the choice itself with candour - as though going to war were a necessary, if unfortunate, stepping stone to his own American dream.

One of the central differences between this generation of deserters and those of the Vietnam era is class, says Lee Zaslofsky, coordinator of the War Resisters Support Campaign in Canada. "Back then we went to university to get deferments from the draft. Now they go into the military to go to university," he says. Troops in the Vietnam war were conscripted; now they are volunteers. Zaslofsky himself went north of the border in 1969.

Poverty is one of two defining factors in recruitment to the military. Not the abject penury of the underclass but the borderline desperation of the aspirant working poor. America's servicemen are better educated than the population at large, with blacks and Hispanics - the two ethnic groups least likely to support the war - the most over-represented in its ranks.

The other factor is patriotism. Not tub-thumping nationalism but the latent, yet strongly-held, belief that the US is a force for good in the world and that its military exists to impose that good when other means have failed. Such views are both so pervasive and dominant in the American psyche that they don't need evidence to sustain them. Support for the troops, regardless of their mission, is an indisputable fact of public discourse. Liberals sport bumper stickers stating: "Support the troops, oppose the war." Flight attendants regularly announce the presence of a serviceman on board to rousing applause.

So when Anderson signed up, he knew there would be a war and, as much as he thought about it at all, he supported it. "I thought I was going to free Iraqi people. I thought I was going to do a good thing. I didn't know anything about the politics of it."

For nearly all of them, the first time this patriotism was put to the test was also the first time they went abroad - to Iraq. Anderson recalls his initial thoughts while on patrol in Baghdad. "I just thought, what are we doing here? Are we looking for weapons of mass destruction? No. Are we helping the people? No, they hate us. What are we working towards, apart from just staying alive? If this was my neighbourhood and foreign soldiers were doing this, then what would I be doing?"

Initially appalled by what he describes as the racism and hatred of some of his fellow soldiers, he said within a few months he was "cocking my weapon at innocent civilians without any sympathy or humanity".

Like Anderson, Camilo Mejia was able to conform for only so long. Mejia worked in a prisoner of war camp in Al Assad. "The prisoners were barefoot, hooded, their hands tied with concertina wire, and we had to soften them up for interrogation," he says. "We had to keep them awake for 48 to 72 hours. They were so tired and occasionally they just couldn't stay awake. Then we would get a sledgehammer and hit the wall so it sounded like an explosion to scare the shit out of them. Sometimes we would put a 9mm pistol to their heads to make them think they were going to be executed. I didn't say anything because I was afraid and everybody else was doing it. Maybe they felt the same as I did, although some of them didn't really mind doing it. But I knew the prisoners were not all terrorists. One man had a rifle to protect his sheep. I said to myself, this guy's innocent. I thought, this is not a prisoner of war camp - this is a torture camp."

Casteel was similarly outraged by events in Abu Ghraib. While training, he would be presented with mock scenarios: "We were told that whatever was written on the file was true," he says. "If it says this person is a genocidal terrorist, then that would be what he was. So you felt justified in what you were doing. But the intelligence we were working with in Iraq was terrible. I interrogated 40 people. I could count on one hand the people who had participated in systematic violence. The rest knew nothing about it. They were taxi drivers or young fathers. Some were involved in tribal defence but that's not systematic violence."

There is a difference between knowing what is wrong, working out what needs to be done and then taking action. To take the fateful step requires maturity and resolve. Anyone who has stayed in an abusive relationship or a demoralising job too long knows what it is to be paralysed by indecision and false hope. It is no different for deserters. Each one now feels certain they made the right choice; but the process by which they made that choice was marked by crippling uncertainty.

"There was a lot of conflict in me," says Mejia. "On the one hand I knew it was brutality. On the other hand I had been preparing to be an infantryman for eight years. I signed a contract. I felt I was not entitled to my opinions. I was worried that I would be perceived as a coward and a traitor. So I thought I would just do it and keep quiet. The sense of community in the military is very strong. You rely on these people in really difficult situations. I didn't want to disappoint them. But these were all justifications you give yourself to avoid the bigger issues. You keep coming back to the bigger picture. What are we doing there? What about the people we are oppressing? In the end I decided there was no way I could justify participating in this war."

Casteel also found himself wavering between extremes. "I was torn," he says. "On the one hand I had my conscience. On the other hand I felt I'm trained for something but I'm watching it from a place of comfort. One week I would feel, this is all absurd, I'm a pacifist and I need to get out. The next week I would think I need to join Special Forces."

And having spoken their own truth, they must then face the consequences. Jail, disparagement, exclusion, ridicule. "Some people called me a traitor," says Anderson. "But I thought, 'You're supposed to support the troops and you're not listening to a word I'm saying.' "

For Ivan Brobeck, 21, joining the marines was his childhood ambition. "As a kid I always wanted to join the military. I was patriotic and I wanted to fight for my country. I thought we had been doing the right thing all along but I didn't really keep up with the news, so I didn't know much about what was going on in Iraq." When he decided he could not return, his mother told him to see a therapist. As he was leaving for Canada with his most important stuff - "My electronics basically" - his mother came to find him to take him back to the base and he had to hide from her.

It's at this point it becomes apparent how young most of these men are. Brobeck may be 21, but with his boyish features he would struggle to get served in a British pub. Most confided not in colleagues or superiors but in their mothers, who in more than one case assisted them in their esape plans.

When Anita Anderson went on talk radio to defend her son, one caller said he should be publicly executed. At the doctor's office in the small conservative town in Kentucky where she lives, her boss called a meeting at which her colleagues, who had previously congratulated her on Darrell's service, said she was not allowed to talk about her son's desertion. "My boss made me sign a paper saying I would resign if the patients started to complain." She got another job. "People say 'Support the troops' but whenever you talk about supporting one individual soldier, they are not interested."

At a picnic for resisters in Fort Erie, Canada, Anderson lies with his head in his wife's lap and his mother sitting alongside him. He is wearing a T-shirt saying AWOL and a broad smile, even though he joined the military in the first place to give himself more options and now he finds himself more trapped than he ever was. His claim for refugee status was denied and he thinks it will be just a few months before he gets his deportation papers. His wife is Canadian, so that might help. The US border is just five minutes' drive away but he can't go back. He says that doesn't bother him. "All those rich people in my country sent me to die for oil and my education," he says. "I don't feel like I want to go back right now. Maybe if things change."

Casteel, meanwhile, has been studying playwriting and non-fiction in Iowa, as well as teaching rhetoric. He recently came to England and performed a monologue entitled Interrogation Room from his play, Returns, about his time at Abu Ghraib and post-traumatic stress disorder. And whatever happened to the young Saudi jihadist who stiffened Casteel's resolve by so reminding him of himself? "I have no clue," says Casteel. "I'm sure that guy's still in prison."