A photo of Greg Kleven, dated April 1967, shows him posing in front of a tin-roofed hooch, wearing an undershirt so stained it matches the sand beneath his feet. In his right hand, he is holding an M-16 rifle. His shaved head is cocked to the left and he's sticking out his tongue in a half smile.

The 18-year-old enlistee is three months into his tour of Vietnam in a Marine Corps Force Reconnaissance company, a special operations unit similar to the Navy SEALs. He looks brash and ready to take on any Viet Cong who cross his path.

"We had all of the difficult missions," Mr. Kleven recalls. "We blew up bridges and parachuted out of planes. Each patrol was like an individual war."

As we talk in his apartment overlooking the Nhieu Loc Canal in Ho Chi Minh City, Vietnam, it's hard to find any trace of that brazen marine in Kleven today. Two decades after leaving Vietnam on a stretcher with a bullet wound to his back, Kleven returned to the country for good in 1991, making him, he says, the first American to live in Ho Chi Minh City after the war.

Today, Kleven's apartment turns into a classroom several times a week when Vietnamese students come to practice their English. Kleven was a trailblazer in Vietnam for English teaching, a field that did not exist when he first returned to the country as a tourist in the 1980s. He and his brother – an Air Force veteran – became the first foreigners granted a government license to teach in Vietnam. His voice can be heard in classrooms across the country on the government's English-language training tapes.

"I wanted to make up for what I had done during the war," Kleven says of his English-teaching career. "I now have a second chance to do things right. I have the chance to be a teacher here instead of a soldier."

Kleven is just one of thousands of American veterans who have returned to Vietnam since the end of one of the most divisive conflicts in American history. In the four decades since the signing of the Paris Peace Accords in 1973, which brought America's direct military involvement in the war to an end, many former soldiers have journeyed there out of curiosity to see a land and people they once fought or to seek closure for a war that continues to weigh on their minds.

While no one knows the precise number of returning vets, most experts put the figure in the tens of thousands. Vietnam Battlefield Tours, just one of dozens of groups that organize trips for former soldiers, estimates it has taken more than 1,000 veterans to the country since the group's founding in 2005. The Vietnamese government says that in recent years more than 400,000 Americans – many of them former military – have visited the country annually.

A few hundred other former soldiers, like Kleven, have moved to Vietnam permanently. Some of these veterans are working alongside their former enemies to address the legacies of the war. They remove unexploded bombs and land mines from old battlefields that are now rice paddies.

They raise money for people who have been diagnosed with disabilities or diseases attributed to exposure to Agent Orange or other herbicides that were sprayed by the United States during the war. And they act as unofficial ambassadors, promoting reconciliation between Americans and Vietnamese as teachers and tour guides.

• • •

American veterans have a long tradition of making pilgrimages to their old battlefields. The journeys serve to memorialize the war and to honor those that lost their lives in battle. Vietnam veterans return to the Southeast Asian country for these reasons, too, but also because they have a need to make sense of a war that remains controversial.

"What makes Vietnam veterans different from World War II veterans who go back is that we lost in Vietnam," says Paulette Curtis, an anthropologist at the University of Notre Dame in South Bend, Ind., who has studied the phenomenon of returning vets. "Veterans that go back to Vietnam are reclaiming their place in history, both in a personal and national sense."

While the men who came home from World War II were celebrated as heroes, Vietnam veterans faced an American public that largely did not support the conflict in Southeast Asia. Added to this, American media coverage of Vietnam dropped off almost entirely after the fall of Saigon in 1975, so veterans had a hard time understanding how their role in the war contributed to the country's well-being.

Kleven recalls the confusion he felt after coming home from Vietnam in 1967. "I kept asking myself, why did we go? What was behind it? I never knew the history of it. So I was searching for all of those things."

The quest for answers drove some veterans to return to Vietnam and connect with the Vietnamese in the 1980s. The trips were difficult in those early years.

The US imposed a trade embargo on Vietnam in 1975 and pulled its embassy staff from the country. Before Kleven first returned to Vietnam in 1988 – 15 years after the peace accords – he was warned by the US State Department not to go. Despite these challenges, veterans made up the largest contingent of Americans visiting Vietnam in the 1980s, Ms. Curtis says.

From the beginning, veterans who returned played a role in improving ties between the two countries. In the absence of formal diplomatic ties, Hanoi reached out to returning American veterans to discuss outstanding war issues, such as missing soldiers and Vietnamese children fathered by American troops. While the US government discouraged these discussions – and some veterans felt too hardened by the war to have any interest in symbolically shaking hands – well-known veterans such as Bobby Muller, then president of Vietnam Veterans of America, took Hanoi up on the offer.

"We see our role as providing a bridge to Vietnam, a conduit to dialogue," Mr. Muller was quoted by The New York Times as saying after a 1984 meeting with Vietnamese Foreign Minister Nguyen Co Thach. "Our Government will not talk to them. So we do represent the only channel with which to exchange information."

When President Clinton announced the normalization of diplomatic relations with Vietnam a decade later, he thanked veterans for supporting reconciliation and moving "beyond the haunting and painful past toward finding common ground for the future."

Veterans in Vietnam today are continuing that process and working to address the past on both the grass-roots and diplomatic levels.

• • •

It's a balmy evening and we're sitting amid the Tiki torches and straw umbrellas of a bar in Da Nang, in central Vietnam. Across the street, ocean waves lap against what US soldiers used to call China Beach. The site was home to a US rest and relaxation center during the war, where soldiers could unwind and play volleyball.

Tonight, I can make out the silhouettes of Vietnamese teenagers playing a casual game of soccer under the moonlight. I had planned on meeting only one veteran, but as the night unfolded, more and more joined us. The expatriate community in Da Nang is close, but the group of US veterans living along the beach is tighter still. Bottles of the local Biere Larue are passed around and the men settle in to reminiscing about their first time going back to Vietnam after the war.

"I flew into Hanoi and I had the jitters in my stomach," Marine vet Chuck Palazzo recalls. "I had some bizarre thoughts that I'm in this database and they're going to see that I was a marine. And they're going to take me away. Crazy thoughts. But to the contrary, big smile on the guy's face, they stamped my passport. I'm in."

The men laugh. Their initial fears of facing their former enemies are still vivid, even after years of having Vietnamese friends and wives. They continue to be astonished by Vietnam's ability to forgive American soldiers for what they did during the war.

"For guys that come back today, they're expecting to find sandbags and bunkers, barbed wire and bullets lying around. But rarely do you find that stuff," says Bill Ervin. The Marine vet has been bringing veterans back to Vietnam since the mid-1990s and runs his own travel agency, Bamboo Moon, out of his home near the beach.

The veterans in Da Nang speak of meeting former North Vietnamese soldiers on the street who embrace them as brothers. And they recall a trip they took together not long ago during which a poor family invited them in to share a meal of coconut worms.

It's clear they feel at home in Vietnam, despite the lingering memories of war.

"I tell people that I was born in Vietnam," Suel Jones, the oldest veteran at the table, tells me. "And they say 'what?' Yeah, I was born here in 1968. Because upon my arrival here every breath I've ever had since has been affected by it in some way or another. Everything I've ever done since leaving Vietnam has been affected by my time here."

Mr. Jones received two Purple Hearts, first for being shot and later for being wounded by mortar fire in Vietnam, but the mental injury he sustained was far more serious. For 30 years he suffered from what was diagnosed as post-traumatic stress disorder (PTSD) and wandered the US, unable to hold a job or make friends. When he finally settled down it was in a cabin in Alaska 60 miles from the nearest town.

In 1995, he traveled to Seattle and underwent a month-long PTSD treatment program at a Veterans Administration hospital. It was during that program he decided to return to Vietnam.

"I was a bush marine and we spent all of our time in the DMZ [demilitarized zone]," Jones says. "I never saw a city. I never saw a Vietnamese that I wasn't shooting at or who wasn't shooting at me. And I knew nothing, absolutely nothing about this country. So I thought, it's time to go back and at least see where in the hell I fought and what the hell happened."

Jones and Mr. Palazzo now live in Da Nang and help run Veterans for Peace's first overseas chapter. Through the organization, they arrange tours for US veterans who are interested in returning to Vietnam and learning more about the legacies of the war. They also spend time visiting those who were exposed to Agent Orange and raising money for the Da Nang Association for Victims of Agent Orange/Dioxin (DAVA), which provides assistance to families in central Vietnam.

• • •

One morning, I drive with Phan Thanh Tien, the vice president of DAVA, to the home of a former Vietnamese soldier whose grandchildren have been affected by dioxin. The house is deep in the countryside in Hoa Vang District, far from the hospitals and markets in downtown Da Nang. We climb a dirt path up a hill to reach the house and pass a cow, munching among the palm trees, that DAVA has given the family.

The house is humble – just a few rooms – and like many Vietnamese homes it lacks a front wall, so the breezes flow through the house freely. Chickens and dogs play in the front yard.

The patriarch of the family, Le Van Dan, is 68 years old. He was a driver for the South Vietnamese Army during the war, a role he says the military forced him to take on. His wife pours me tea while one of his grandsons examines my notebook and pen.

He is 15 years old, but his developmental disabilities make him look much younger. A second grandson lies motionless on a bed in the other room. His legs are skinny and unable to keep him upright. When I speak to him, his eyes move, but he doesn't say a word. They tell me that he can't understand.

Mr. Dan remembers seeing the white clouds of herbicide being sprayed by US planes a few miles from his home during the war. He thought it was water at the time. American veteran Palazzo remembers those clouds, too. He served as a reconnaissance specialist not far from Dan's house and was told by his commanding officer that the chemical being sprayed was pesticide. But something about the manure-smelling chemical didn't make sense.

"I paid more attention and I realized that this stuff is wilting the leaves, and the trees are just crumbling down," Palazzo recalls. "Mosquito repellent doesn't do this."

The US sprayed nearly 20 million gallons of herbicides in Vietnam, Laos, and Cambodia as part of Operation Ranch Hand during the war against North Vietnam. Of these herbicides, Agent Orange is the most deadly. The chemical contains a large amount of dioxin, a toxic compound that takes centuries to break down.

Since 1991, the US Department of Veterans Affairs has recognized that exposure to dioxin can cause certain cancers, diseases, and birth defects. American veterans who served in the Vietnam War and have conditions linked to dioxin exposure can receive medical benefits and disability compensation from the US government.

But Hanoi has long complained that the 4.8 million Vietnamese that it estimates were exposed to dioxin during the war have not received equivalent benefits from Washington. Moreover, the former US bases where herbicides were once stored, mixed, and loaded into planes have not been properly cleaned. Researchers at the Vietnam Public Health Association estimate that 90 percent of new dioxin poisoning cases occurring today in Vietnam are due to the consumption of food and water contaminated by dioxin that has leaked out of bases abandoned by the US four decades ago.

The lack of response by the US government is largely what fueled Palazzo's decision to return to Vietnam and address the outstanding legacies of the war.

"I felt a tremendous amount of guilt," Palazzo says. "While I was in Vietnam during the war I had a dream. It stuck in my mind for years that I wanted to come back, and I wanted to do something positive after all of the destruction that we did. So years later, the time was right and I decided to return."

Last year, Palazzo visited Dan's family with a group of Americans in Vietnam for the annual Veterans for Peace tour. Dan was moved by the meeting.

"It encouraged us and gave us the strength to continue taking care of our grandchildren," he says. "If the US veterans did not come back to Vietnam they would not understand what happened here and wouldn't sympathize with us. It's very, very important for the Vietnamese to see and hear from American veterans."

• • •

The effects of such sympathy are often most visible at the local level, in the new schools veterans have built and the freight containers full of wheelchairs they have donated to hospitals and orphanages. Yet the work of American veterans in Vietnam can be felt at the diplomatic level as well.

After years of campaigning by activist veterans, the US began removing dioxin from the soil of a former US airport in Da Nang in 2012. The project involves digging up contaminated soil and heating it to high temperatures to destroy the toxin. The US Agency for International Development estimates that 73,000 cubic meters of soil will be remediated at the airport, enough to fill 29 Olympic-size swimming pools.

"The US would have been much too ready to totally ignore land mines, unexploded ordnance, Agent Orange, and the tragic legacy of the war if we had let them," says Chuck Searcy, a US Army veteran who has lived in Hanoi since relations were normalized in 1995. "But because of the presence here of veterans and our attention to those issues, the US has had to be accountable for those things."

Mr. Searcy has seen firsthand the power of veterans to encourage reconciliation through his work on land mines and unexploded ordnance. The US dropped an estimated 7.8 million tons of ammunition on Southeast Asia during the war, more than it used on both Germany and Japan during World War II. Bombs that failed to detonate on impact became de facto land mines. The Vietnamese government estimates that more than 100,000 people have been killed or injured by unexploded ordnance since the war ended.

Searcy was stationed as an intelligence officer in Saigon during the 1968 Tet Offensive, North Vietnam's largest assault in the war. He returned to Vietnam in the 1990s to serve as the representative for the Washington, D.C.-based Vietnam Veterans of America Foundation and then later helped found Project Renew, a joint Vietnamese-international group that deals with the explosive remnants of war.

Searcy says that his status as a veteran has given him access to people in the Vietnamese military and government that other Americans cannot reach.

"When I first met with the Ministry of Defense in 1996, they had never had a meeting with an NGO [nongovernmental organization] before, and they did not deal with NGOs period," Searcy recalls. "The only reason they gave me a meeting, they said, was because I was an American veteran. They said, you know what we suffered through because you suffered the same thing. We're brothers."

This access proved useful a few years later when Searcy helped resolve a disagreement between Washington and Hanoi over a donation of de-mining equipment. The Vietnamese government was concerned about demands attached to the donation, Searcy says, preventing a deal from moving forward.

After speaking with his military contacts in Vietnam and in the US, Searcy was able to help bridge the gulf between the parties and bolster Hanoi's confidence in the gift. A multimillion-dollar donation package was signed soon after.

Four decades after the Paris Peace Accords, many of the men who went to Vietnam as teenagers are now in their 60s and 70s. Age is posing new challenges for those living in the Southeast Asian country.

Some American veterans in Vietnam are considering moving back to the US, where they can have access to treatment at VA hospitals and have the support of family members. Others are thinking about winding down their humanitarian projects as the work of fundraising becomes too taxing.

Yet as veterans in Vietnam contemplate leaving, many veterans in the US who are now retired are considering returning to the country for the first time.

"Veterans can afford to go to Vietnam now that their kids are gone and they have more free time on their hands," says Ed Stiteler, president of Vietnam Battlefield Tours and a Vietnam veteran himself. "Most of the people we're dealing with have put a lot of thought into going to Vietnam and are looking for closure and healing."

A similar dynamic is also driving many former Vietnamese soldiers back to their own battlefields, increasing the likelihood of chance connections between the former enemies.

"Commemorating the war together is one of the most important ways in which veterans are able to work through the past," says Christina Schwenkel, an anthropologist at the University of California, Riverside. "Veterans who meet today in Vietnam are sharing their sorrow and trying to move forward together."

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Decades after they first went to war in Vietnam, many of them are finally making peace with the past.

•Nissa Rhee is writing a book about American veterans who have returned to Vietnam to help overcome the legacies of the war, reconcile the past with the present, and turn enemies into friends.