LEEDS, Mass. - Peter Mohan traces the path from the Iraqi battlefield to this lifeless conference room, where he sits in a kilt and a Camp Kill Yourself T-shirt and calmly describes how he became a sad cliche: a homeless veteran.

There was a happy homecoming, but then an accident - car crash, broken collarbone. And then a move east, close to his wife's new job but away from his best friends.

And then self-destruction: He would gun his motorcycle to 100 mph and try to stand on the seat. He would wait for his wife to leave in the morning, draw the blinds and open up whatever bottle of booze was closest.

He would pull out his gun, a .45-caliber, semiautomatic pistol. He would lovingly clean it, or just look at it and put it away. Sometimes place it in his mouth.

"I don't know what to do anymore," his wife, Anna, told him one day. "You can't be here anymore."

Peter Mohan never did find a steady job after he left Iraq. He lost his wife - a judge granted their divorce this fall - and he lost his friends and he lost his home, and now he is here, in a shelter.

He is 28 years old. "People come back from war different," he offers by way of a summary.

This is not a new story in America: A young veteran back from war whose struggle to rejoin society has failed, at least for the moment, fighting demons and left homeless.

But it is happening to a new generation. As the war in Afghanistan plods on in its seventh year, and the war in Iraq in its fifth, a new cadre of homeless veterans is taking shape.

And with it come the questions: How is it that a nation that became so familiar with the archetypal homeless, combat-addled Vietnam veteran is now watching as more homeless veterans turn up from new wars?

What lessons have we not learned? Who is failing these people? Or is homelessness an unavoidable byproduct of war, of young men and women who devote themselves to serving their country and then see things no man or woman should?

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For as long as the United States has sent its young men - and later its young women - off to war, it has watched as a segment of them come home and lose the battle with their own memories, their own scars, and wind up without homes.

The Civil War produced thousands of wandering veterans. Frequently addicted to morphine, they were known as "tramps," searching for jobs and, in many cases, literally still tending their wounds.

More than a decade after the end of World War I, the "Bonus Army" descended on Washington - demanding immediate payment on benefits that had been promised to them, but payable years later - and were routed by the U.S. military.

And, most publicly and perhaps most painfully, there was Vietnam: Tens of thousands of war-weary veterans, infamously rejected or forgotten by many of their own fellow citizens.

Now it is happening again, in small but growing numbers.

For now, about 1,500 veterans from Iraq and Afghanistan have been identified by the Department of Veterans Affairs. About 400 of them have taken part in VA programs designed to target homelessness.

The 1,500 are a small, young segment of an estimated 336,000 veterans in the United States who were homeless at some point in 2006, the most recent year for which statistics are available, according to the National Alliance to End Homelessness.

Still, advocates for homeless veterans use words like "surge" and "onslaught" and even "tsunami" to describe what could happen in the coming years, as both wars continue and thousands of veterans struggle with post-traumatic stress.

People who have studied postwar trauma say there is always a lengthy gap between coming home - the time of parades and backslaps and "The Boys Are Back in Town" on the local FM station - and the moments of utter darkness that leave some of them homeless.

In that time, usually a period of years, some veterans focus on the horrors they saw on the battlefield, or the friends they lost, or why on earth they themselves deserved to come home at all. They self-medicate, develop addictions, spiral down.

How - or perhaps the better question is why - is this happening again?

"I really wish I could answer that question," says Anthony Belcher, an outreach supervisor at New Directions, which conducts monthly sweeps of Skid Row in Los Angeles, identifying homeless veterans and trying to help them get over addictions.

"It's the same question I've been asking myself and everyone around me. I'm like, wait, wait, hold it, we did this before. I don't know how our society can allow this to happen again."

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Mental illness, financial troubles and difficulty in finding affordable housing are generally accepted as the three primary causes of homelessness among veterans, and in the case of Iraq and Afghanistan, the first has raised particular concern.

Iraq veterans are less likely to have substance abuse problems but more likely to suffer mental illness, particularly post-traumatic stress, according to the Veterans Administration. And that stress by itself can trigger substance abuse.

Some advocates say there are also some factors particular to the Iraq war, like multiple deployments and the proliferation of improvised explosive devices, that could be pulling an early trigger on stress disorders that can lead to homelessness.

While many Vietnam veterans began showing manifestations of stress disorders roughly 10 years after returning from the front, Iraq and Afghanistan veterans have shown the signs much earlier.

That could also be because stress disorders are much better understood now than they were a generation ago, advocates say.

"There's something about going back, and a third and a fourth time, that really aggravates that level of stress," said Michael Blecker, executive director of Swords to Plowshares," a San Francisco homeless-vet outreach program.

"And being in a situation where you have these IEDs, everywhere's a combat zone. There's no really safe zone there. I think that all is just a stew for post-traumatic stress disorder."

Others point to something more difficult to define, something about American culture that - while celebrating and honoring troops in a very real way upon their homecoming - ultimately forgets them.

This is not necessarily due to deliberate negligence. Perhaps because of the lingering memory of Vietnam, when troops returned from an unpopular war to face open hostility, many Americans have taken care to express support for the troops even as they solidly disapprove of the war in Iraq.

But it remains easy for veterans home from Iraq for several years, and teetering on the edge of losing a job or home, to slip into the shadows. And as their troubles mount, they often feel increasingly alienated from friends and family members.

"War changes people," says John Driscoll, vice president for operations and programs at the National Coalition for Homeless Veterans. "Your trust in people is strained. You've been separated from loved ones and friends. The camaraderie between troops is very extreme, and now you feel vulnerable."

The VA spends about $265 million annually on programs targeting homeless veterans. And as Iraq and Afghanistan veterans face problems, the VA will not simply "wait for 10 years until they show up," Pete Dougherty, the VA's director of homeless programs, said when the new figures were released.

"We're out there now trying to get everybody we can to get those kinds of services today, so we avoid this kind of problem in the future," he said.

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These are all problems defined in broad strokes, but they cascade in very real and acute ways in the lives of individual veterans.

Take Mike Lally. He thinks back now to the long stretches in the stifling Iraq heat, nothing to do but play Spades and count flies, and about the day insurgents killed the friendly shop owner who sold his battalion Pringles and candy bars.

He thinks about crouching in the back of a Humvee watching bullets crash into fuel tanks during his first firefight, and about waiting back at base for the vodka his mother sent him, dyed blue and concealed in bottles of Scope mouthwash.

It was a little maddening, he supposes, every piece of it, but Lally is fairly sure that what finally cracked him was the bodies. Unloading the dead from ambulances and loading them onto helicopters. That was his job.

"I guess I loaded at least 20," he says. "Always a couple at a time. And you knew who it was. You always knew who it was."

It was in 2004, when he came back from his second tour in Iraq with the Marine Corps, that his own bumpy ride down began.

He would wake up at night, sweating and screaming, and during the days he imagined people in the shadows - a state the professionals call hypervigilence and Mike Lally calls "being on high alert, all the time."

His father-in-law tossed him a job installing vinyl siding, but the stress overcame him, and Lally began to drink. A little rum in his morning coffee at first, and before he knew it he was drunk on the job, and then had no job at all.

And now Mike Lally, still only 26 years old, is here, booted out of his house by his wife, padding around in an old T-shirt and sweats at a Leeds shelter called Soldier On, trying to get sober and perhaps, on a day he can envision but not yet grasp, get his home and family and life back.

"I was trying to live every day in a fog," he says, reflecting between spits of tobacco juice. "I'd think I was back in there, see people popping out of windows. Any loud noise would set me off. It still does."

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Soldier On is staffed entirely by homeless veterans. A handful who fought in Iraq or Afghanistan, usually six or seven at a time, mix with dozens from Vietnam. Its president, Jack Downing, has spent nearly four decades working with addicts, the homeless and the mentally ill.

Next spring, he plans to open a limited-equity cooperative in the western Massachusetts city of Pittsfield. Formerly homeless veterans will live there, with half their rents going into individual deposit accounts.

Downing is convinced that ushering homeless veterans back into homeownership is the best way out of the pattern of homelessness that has repeated itself in an endless loop, war after war.

"It's a disgrace," Downing says. "You have served your country, you get damaged, and you come back and we don't take care of you. And we make you prove that you need our services."

"And how do you prove it?" he continues, voice rising in anger. "You prove it by regularly failing until you end up in a system where you're identified as a person in crisis. That has shocked me."

Even as the nation gains a much better understanding of the types of post-traumatic stress disorders suffered by so many thousands of veterans - even as it learns the lessons of Vietnam and tries to learn the lessons of Iraq - it is probably impossible to foretell a day when young American men and women come home from wars unscarred.

At least as long as there are wars.

But Driscoll, at least, sees an opportunity to do much better.

He notes that the VA now has more than 200 veteran adjustment centers to help ease the transition back into society, and the existence of more than 900 VA-connected community clinics nationwide.

"We're hopeful that five years down the road, you're not going to see the same problems you saw after the Vietnam War," he says. "If we as a nation do the right thing by these guys."

© 2008 The Associated Press