A waterfront breeze whips at John Prendergast and Michael Mattocks as they walk down to the Potomac River on a sunny spring afternoon. The wind plays decidedly more havoc with Prendergast — a tall white man with flowing, Fabio-class hair — than it does with Mattocks, a stocky African American with a shaved head and a tight inch of beard along the ridge of his jaw.

Prendergast, 48 and a former member of President Bill Clinton’s national security team, unconsciously rakes the tresses from his face every few seconds as he looks over the riverside where he and Mattocks used to fish.

Mattocks, 34 and a former drug dealer, squints in the sun, chin up, coolly watching a team of suburban high-schoolers trot their expensive fiberglass racing scull down the ramp at Thompson’s Boathouse.

Funny, they don’t look like brothers.

“You remember we was down here one day, and I laid my rod down with the hook in the water and zip, that thing was gone!” Mattocks says with a laugh, his eyes suddenly alight with a sweet memory from a sour childhood. He never made it to any high school, much less one that boasted a rowing team. Before he was kicked out of seventh grade, he used to skip lunch to sell crack cocaine on North Capitol Street.

“Oh, man,” says Prendergast, grasping Mattocks by the biceps. He speaks with quick touches and punching gestures. An athlete at fine high schools, Prendergast attended Georgetown University and went on to a career as a human rights advocate that would make him a companion of presidents and movie stars. But back then, when the rod and reel plunked into the river, “I’m sure I was just horrified I was going to have to buy some other [expletive] thing I had no money for.”

They don’t even look much like friends.

But Michael and J.P., denizens of two very different Washingtons, are at this favorite spot celebrating a relationship that has spanned 25 years and produced a harrowing record of violence, despair and, finally, redemption. After a chance meeting in a homeless shelter in 1984, an aimless 21-year-old activist and a homeless 7-year-old city kid effectively declared each other brothers for life. And then, life happened.

One would spend years, driven by a brutal family estrangement, living through a frantic, corrosive search for social justice that has stretched from the District to Darfur, leaving his psyche and a first marriage in tatters.

The other would find his footing in the mayhem of the crack epidemic that raged through urban Washington in the 1990s, becoming a dealer before he could drive and seeing multiple friends and family members dead on bloody sidewalks.

“I’ve been telling John for years that we ought to write a book,” says Mattocks, who finally renounced “the life” in 1999. He now lives in Gaithersburg with his wife and five sons and drives a shuttle bus between Shady Grove Metro and the King Farm development.

And now they have. Their twin memoir, “ Unlikely Brothers ,” written with help from magazine writer Dan Baum, is due out May 17.

It’s going to be a busy summer for both of them.

Prendergast — who has escorted the likes of Angelina Jolie and George Clooney through African war zones — is getting married next month up at Mia Farrow’s house in Connecticut.

Mattocks — who used to rob drug addicts for the fun of it, took part in many an urban gunfight and once shot his cousin in the leg in a family spat — will be there.

“My 6-year-old said, ‘Daddy, I’m gonna wear a tuxedo to John’s wedding,’ ” Mattocks says with pride. “ ’Course we gonna be there! Gonna load up the minivan and drive to Connecticut.”

Hey, you don’t miss your brother’s wedding. Prendergast didn’t miss Mattocks’s, 10 years ago. His own parents came down for it, two aging white Philly suburbanites dancing hip-hop at a community center in Silver Spring.

Family is family, no matter how elective. Or explosive.

Mattocks had it bad from the very beginning. Born into a combative family in Southeast Washington, he says one of his first memories was of the time Aunt Francine’s husband beat her to death with an iron and Sheetrocked her body behind a wall. His father was not around, and the kids often had to feed themselves, usually cold cereal.

But the family was as close as it was chaotic. Michael’s mom strained mightily to keep the centrifugal forces of poverty and addiction from spinning them apart. They spent years homeless, dragging Hefty bags of belongings from one shelter to another. But they still went to school. The other kids teased them about the shabby sweatsuits they wore.

His little brother (and constant companion) James had a temper, but Michael was moon-faced and cheerful. So peppy, in fact, that he caught the eye and captured the heart of a young Georgetown dropout who had come by to visit a friend on the shelter staff. Prendergast was 21. Michael was 7.

Michael and James were digging coins out of the couch, and on a whim, Prendergast asked their mother if he could take them out for a break from the bleak. They went to the library and a McDonald’s. They loved the attention; he was amazed at their sunny resilience.

“I mean, these boys had nothing and yet radiated with life and sunshine,” he writes. If anybody ever needed a steady big brother . . .

He came back a few days later, sprang them for another outing, and a routine was born. It was an ersatz fraternity forged in other libraries, more McDonald’s. But their favorite thing was the fishing behind the Watergate.

When they changed shelters, Prendergast tracked them down. When they crashed in dicey flophouses, he bounded up the steps and whisked them away through suspicious and sullen stares.

“He used to find us,” Mattocks recalls with wonder. “He’d come around in that old station wagon and it was like we were seeing God.”

Prendergast was working on some savior fantasies of his own at that time.

The oldest son of a gregarious Irish Catholic frozen food salesman, J.P. had a solid family life, but it went off the rails shortly before high school in Philly. For reasons he is still trying to understand, his father, a favorite of the neighborhood kids and a popular raconteur, began targeting J.P. with an explosive rage. Even though he brought home good grades and stayed away from drugs, adolescence deteriorated into a sustained screaming match whenever his father was in from the road. When there was nothing left to shout, they stopped speaking entirely. For decades.

Around the same time, J.P. developed disfiguring acne that would mar his face for years. The star student and standout athlete became “the lizard,” an estranged recluse, making his grades but hiding behind long hair, alienated from a father who charmed everyone else.

At Georgetown, the persecutions of his youth turned him into a manic champion of the persecuted. He dropped out and hitchhiked the country on a personal save-the-world tour. It was prison reform and the death penalty in California, urban poverty in Philadelphia. Soon it would be the mother lode of suffering: war and famine in Africa. But along the way it also become a couple of homeless kids back in the District.

The outings continued for years. He took the boys to Philadelphia for weekends, once for the whole summer. But their time together became less regular as J.P. made his first trips to Africa. In the same headlong way he had “adopted” two little brothers, he rode one-way tickets into the refu­gee camps of Somalia and Ethiopia. They needed saviors, too.

Michael, meanwhile, was growing up in a sea of drugs. And a few fishing trips each year weren’t going to keep him dry.

Crack cocaine flooded the streets and filled the house in the 1400 block of North Capitol Street where Michael came to live with a rotating cast of cousins and friends. Still cherubic and small, Michael learned that one way to keep kids from teasing him about his worn clothes was to hang with the young men who “pumped” the little white rocks for higher-up dealers.

Michael was 9 when he and his best friend “Little Charles” began making fake crack from baking soda and Orajel and selling it to desperate junkies. Soon, a dealer fronted him some of the real thing. He sold 25 packets in half an hour, pocketing $500.

Word went around, Michael could pump! Soon he controlled his own territory.

“I was the kingpin of the block when I was 14,” he said last month, standing in front of his old house. It now bears the sign of a photography studio, but looks sealed and unlived in.

He and Prendergast have driven over to the old place after their visit to the river. “I used to set up a little table right over there,” Mattocks said.

Little Charles gave him his first gun, a .22. He didn’t wear those sweatsuits anymore.

Nobody picked on him.

Mattocks will say it still: He liked the life. He liked the money it brought, the security for his desperate family. He liked the respect he gained, the goods that are yours for the plucking when you carry a Colt .38 and everybody knows that killers have your back. When he admired some dude’s jacket, it was his.

“This neighborhood was America’s nightmare,” he says. “Trust me, it was bad.” He pauses to take a drag on his Newport and shakes his head, can’t help smiling. “But I used to love the [expletive] I’d do.”

But one day he heard a single Pop! on the sidewalk and came out to see Little Charles leaning against the wall with a hole in his face, the blood raining like a fountain.

It was an accident, another kid playing with another gun. But he started drinking more after that, staying stoned all the time. He got harder, took up with the baddest of the bad, a guy named Cool who let fly with an Uzi one day at a McDonald’s just to mess with some cops.

Prendergast admits it now: He ignored the obvious for years. Even when Michael’s mother would buttonhole him, warn him that the boy was slipping, he would brush it off. Michael assured him he was clean, and it was just easier to believe him.

Besides, he was disappearing ever deeper into his devotion to the hardest cases in Africa. He was in growing demand among advocacy groups for his willingness to parachute into the worst places and his skill at sending back a cogent analysis.

When he did ride over to pick up the boys, Michael eventually began to hang back. Prendergast would collect James and soon started including their younger brothers Tyrell and David. Michael would watch from the window.

“When they’d come home I’d be like, ‘What did you do? Where did you go?’ ” Michael says. “I really wanted somebody to pull me out it, I wanted John to pull me out it. But I was too cool for school.”

Prendergast rose in the human rights ranks. Speaking at a conference at Princeton University, he caught the attention of Susan Rice, then Bill Clinton’s senior adviser on African security. A few months later Prendergast found himself with a six-month fellowship in the White House.

“He was different in that he had served in the field, he had real hands-on experience,” recalls Rice, now the U.S. ambassador to the United Nations. She said she remembers his stories of Michael and James: “He called them his little brothers. It was something obviously very important to him.”

When the fellowship ended, he landed a real job and a big one, director of African affairs for the National Security Council. He was away for months at a time, worked for two years brokering peace talks between Ethi­o­pia and Eritrea. He didn’t see much of his wife, a corporate manager he married in 1991. Their marriage wouldn’t survive his obsession with overseas work.

And he saw even less of the Mattocks boys. While he was draping a security clearance over his frayed suits for work in the White House compound, Michael was letting off rounds from his “four-five” at competing dealers.

“I just wasn’t there for him when he needed it,” Prendergast says.

After his White House stint, Prendergast’s profile only grew. He mastered the advocate’s art of exploiting celebrity to advance a cause, shepherding Jolie around Congo, Clooney through Sudan. He and “Hotel Rwanda” star Don Cheadle teamed up to write two books on genocide.

“There is no question that John is a warped individual,” Cheadle said by e-mail. “I often ask him what it is that drives him to do what he does. I’m not sure I ever get a complete answer. John obviously believes that some of the biggest problems we face on this planet can be solved with commitment and creativity. It’s an infectious philosophy.”

Michael ended up staying in the business for almost a decade. He took James onto the streets with him, at first just to keep his hotheaded brother under his gaze but eventually as an active partner. They moved up to Georgia Avenue, near Walter Reed Army Medical Center, and established new territory there. Michael was a godfather in the neighborhood, handing out cash, bribing police, commanding fealty. He was arrested a few times but never served any hard time.

And the killing never stopped. Cool was shot in the stomach. Michael’s mother’s boyfriend was stabbed to death on their front steps. His girlfriend’s father, a straight-arrow crane operator, was killed by a stray bullet. His brother Tyrell hanged himself.

And one day James, soon after renouncing the life and joining a church, got into an argument over a thrown firecracker. He was gunned down the next day.

At one point it got to be too much. Michael borrowed a nickel-plated .38, fully loaded with hollow-points, and put it to his temple. Click. It misfired; he passed out. When he woke up, he got back to business.

“I never told J.P. about that,” he writes. “I never told anybody about it until right now.”

What finally turned him were the kids. His girlfriend, Nikki Jackson, had one baby when they got together and soon they had two more together, all boys. When he looked into those tiny eyes, all the stress and death and uncertainty began to overwhelm him. Suddenly, he really didn’t want to wind up in a cell or a coffin.

He just stopped. It was over. They lived for months on the cash he kept jammed into a metal trash can in their Silver Spring house. When it finally ran out, he didn’t have a clue what to do next, how to make an honest living. He called his big brother.

At first it was hard to make Prendergast believe how deeply Michael had been in the drug world, so entrenched was the denial of a teetotaling peace activist who had never even tried marijuana. But he dove in again, giving the young family money, helping Michael find work in the cafeteria at Washington Hospital Center.

At first Michael hated a weekly paycheck that added up to less than he made in 10 minutes on the street. But Nikki pressed him to keep on. He grew to take pride in regular work and stayed 11 years at the hospital before getting his commercial driver’s license.

They were married in 2001 and now have five sons. The kids are good students, do sports and thea­ter (Little Mike was a Munchkin in “The Wiz” last year), and Michael and Nikki always show up to cheer. He thinks constantly about making things different from his own youth, the shelters, the Hefty bags, the guns.

“I tell my boys I love them every day,” he says. “I know they are the reason God kept me alive.”

Prendergast has also been exorcising the ghosts of youth. A few years ago, his father died, not long after they had enjoyed a tentative reconciliation. Soon he found himself battling a major depression, the long-delayed impact of childhood trauma and a career enmeshed in endless suffering and frequent danger.

He found redemption in three places: a deepening spirituality, the love of the civil rights lawyer he will marry next month and . . . Michael. His brother-turned-drug-dealer-turned-model-dad has taught him a lot about selflessness, he says.

“Through his strength, his heart and his success as a father, he is illuminating a path that I have rejected for years but that I am now thinking about walking myself,” Prendergast writes.

These days, it’s a healthier peace activist who continues his work in the war zones and a happier former drug dealer who lives safe and peaceful in the suburbs.

But they are together now, braced to have their secrets bared to a world that knows little of their darker sides. They stand on their old riverbank, thinking of all the water that has flowed by since two unlikely brothers cast their lots together at a more innocent age.

“We have been through a lot together, haven’t we?” says the one.

“Yes, we have, brother,” says the other. “Yes, we have.”