This is a story about lost and broken things, the rubble from which the phoenix-in this case a C-130 military transport-rises over the Caribbean Sea on a spotless day in September 2008. From 30,000 feet, the surface of the water glitters below like jagged glass, shooting spears of light. The plane stalks east, running parallel to the northern coast of Cuba twenty miles off. On board, Major Eric Montalvo is wedged in a seat, thinking, What the fuck have I gotten myself into now?

A month ago, he'd been working at Parris Island, South Carolina, capping a distinguished career during which he'd won more than 95 percent of his cases. He'd recently bought a big house with a huge kitchen and a fountain out back for his wife and two boys-and had begun to turn his attention to finding a civilian job. And then an e-mail pinged his in-box. Copied to a couple of hundred Marine lawyers, it called for applications to help with the military commissions trials at Guantánamo. Montalvo responded impulsively, stirred by the call to duty. Within a couple of hours, he received word. His retirement had been pulled: He was going to Washington, D.C.

The timing was terrible. The real estate market was imploding, the house couldn't be sold, and Montalvo was forced to leave his family for an indeterminate amount of time. Still, there was worse to come. When he found out he'd been placed on the defense side-when he realized that he'd actually be defending the terrorists-he was stricken. The phone started ringing, colonels he knew on the line repeating the same mantra: "This isn't going to be good for your career, Major." Then the call with his parents. On September 11, Montalvo's uncle Tony had responded with his Harlem fire company to Ground Zero, and Montalvo's parents believed it was black lung that killed him not long after.

Please don't do this, Montalvo's mother told him.

Now the transport sweeps wide around Cuba's eastern tip, an arid land of organ-pipe cacti and big loping rodents called banana rats. Below is Guantánamo Bay itself (a flume of blue water mushrooming inland, teeming with turtles and parrot fish) and then the naval base, a scattering of roads, buildings, and low-slung homes that accommodate the 6,000 troops here. Montalvo's first impression is how foreboding the rocky shoreline seems, how moonlike the landscape. A Caribbean Alcatraz. Somewhere down there, too, are the cages that contain "the worst of the worst"-as Donald Rumsfeld labeled them-alleged Al Qaeda terrorists. Montalvo's stomach burns a little with the thought that he might have to collude with any of them, in any manner. A self-described superpatriot and son of blue-collar parents (mother a hairdresser, father a cargo man for AeroMexico), he grew up in Queens, a skinny Puerto Rican scrapper, then joined the Marines at 18 and morphed into "Mad Dog," his gonzo jarhead persona. Soon he was touted for Officer Candidates School and afterward went on to law school at Temple, emerging with gravitas as this slightly fattened-up (five feet nine, 220 pounds) lawyer of laser logic and indignant rage, trimmed beard flecked gray, and bad attorney's back.

When he first found out he'd been assigned to the defense side, he went and spoke to Colonel Morris Davis, the chief prosecutor for the commissions at the time, and the colonel asked Montalvo if he thought he could handle it. The job would get very personal, he said, but it was also the most meaningful kind of work because it was all about the Constitution. And this is how Montalvo buttressed himself in the face of so many doubters, repeating it back to them. "If I'm going to fight the fight for America," he'd say, "dead center on the Constitution is where I want to be."

He soon finds himself on the top step of the military commissions building, gazing down on a makeshift tent city and sweating through his cammies in the heat. He's been assigned the separate cases of two detainees, and enters a small interrogation room where the first, a Yemeni named Ali al-Bahlul, is chained and shackled to the floor. The detainee is surprisingly lithe, a handsome man with close-cropped hair who speaks impeccable English. He's one of Osama bin Laden's former media operatives, most famous for having made a two-hour video celebrating Al Qaeda's attack on the USS Cole. It's one of the jihad movement's all-time greatest hits, and al-Bahlul is also among the most doctrinaire, having been locked away in solitary for years. As they sit face-to-face, al-Bahlul asks why the Marine is trying to be so accommodating. "Don't you know I'm your enemy?" he says. Montalvo responds that, legally speaking, he feels that a First Amendment argument can be made on his behalf, but al-Bahlul interrupts, jangling his chains. "Don't you know that if that door were opened and we both were out there free, I'd kill you?" Nothing has prepared Montalvo for this kind of venom, but his reaction is visceral. He leans forward and says, "Don't you know that if that door were open and we both were free, I'd kill you first?" When it's over, Montalvo leaves the room shell-shocked, thinking, Jesus, how can I defend that?

It's not just al-Bahlul. He feels it all around: that what's really being played out here is a clash of civilizations. Even the setting, the harsh sun and roiling ocean, evokes a desolation, the sense that in this otherworld there's been an allowance made for some unsayable human brutality. Eye for an eye. Montalvo goes back to his tent, gets some grub, sleeps in double sleeping bags because the air-conditioning blows an arctic frost all night. The next day he's introduced to his second client, Internal Security Number 900. The detainee is said to have committed an attack against two Special Forces soldiers in a marketplace in downtown Kabul, a brazen assault with a grenade that left the soldiers badly maimed but alive.

By Michael Paterniti