"Welcome to this dam place," said Maj. Mark Winn. He couldn't resist. "Take all the dam pictures you want."

Some 130 miles northwest of Baghdad, Winn and about 400 other U.S. Marines live inside a dam. It is an incongruous assignment in a country that conjures images of blowing sand and hot desert. But here they are, bunked in the former offices of a massive, Russian-built hydroelectric dam on the Euphrates River.

When the U.S. military planned its invasion of Iraq, strategists pondered the Haditha Dam, recalling oil wells set ablaze by then-President Saddam Hussein in his scorched-earth retreat from Kuwaiti during the 1991 Persian Gulf War. Sabotage to this dam could unleash walls of water from Lake Qadisiya on towns and villages for hundreds of miles. It would cripple the country's electricity supply. It would destroy vast fields of irrigated farmland.

So Army Rangers were dropped in early in the war, and they seized the dam April 1, 2003, eight days before the fall of Baghdad. It has been in U.S. hands ever since. The 1st Battalion of the 8th Marine Regiment is the latest outfit assigned to guard it, arriving here about one month ago from Camp Lejeune, N.C. They share duties with 150 Azerbaijani troops, part of the U.S.-led military coalition in Iraq.

"It's the nicest place in Iraq," said Winn, 41, a 16-year Marine Corps veteran with a ready smile. "It's 10 degrees cooler than anyplace else. We don't get the dust here. And from the top of the dam, we have the best view in the country."

From there, 10 stories above the water, Marines watch the Euphrates weave its serpentine course through dusty desert ridges. On its way, the river nourishes periodic explosions of green abundance, oases of date palms, grass and vegetable gardens.

The dam constantly vibrates with the low rumble of turbines, giving the place the feel of a ship. The Marines' bunks are jammed into linoleum-floored rooms rigged with fluorescent lights. They trudge up and down the 286 stairs, hang their wash on the building's terraced levels and watch huge carp feed in the froth of the spillway discharge. Terns and gulls swoop in the air currents.

"I never thought I'd be on a boat in Iraq," said Sgt. Joseph Wright, 24, from Standish, Me., as he cruised Lake Qadisiya late one night. In the cockpit of a patrol boat bristling with armament, Wright throttled up the twin 300-horsepower jet motors, making the boat rear on its stern and sending up a roostertail wake.

"It can get a little repetitious," he said of the patrol, as he watched for boats approaching the dam. "But we are happy to do it. For a group of people who are intent on destabilizing Iraq, all it would take is a boat loaded with explosives."

The dam was completed in 1987 atop a small island that was home to Hassan Yahier Hassan, now the dam manager. Hassan said his father was heartbroken when his land was taken, and "even today," he added, "we are sad to recall our good life there, when we could just take whatever fruit and fish we wanted."

But Hassan, 53, recognizes the importance of the dam. It is the second-largest of eight hydroelectric dams in Iraq. Its output -- 670 megawatts when the water flow is strong -- serves to stabilize the entire Iraqi power grid, increasing output when needed, reducing it when not. And irrigation canals that feed a wide swath of rice fields south of the dam depend on a steady flow of water, held in the big lake during winter and spring when snows melt in the Turkish mountains.

The Marines patrol not only the lake and river but also the long desert highways, searching for roadside bombs. A contingent patrols an ammunition dump. Another group lives with and trains the Iraqi National Guard in the town of Haditha, 10 miles south of the dam. With all the duties, sleep is often forgone. The Marines don't complain much, except about the chow.

"I volunteered to be in the Marine Corps," said 1st Lt. James Haigh, 26, of Kankakee, Ill. "I never expected to be on a dam in Iraq watching green trees, but I think it's important. What we are doing is helping out the people in Iraq."

That feeling is not always reciprocated. Last month, two suicide car bombs were set off at the Haditha police station and the National Guard post shared by the Marines. Ten Iraqis were killed. Predictably in this land of conspiracy theories, the Americans were blamed. The town leaders regularly meet with Winn to ask him when he will leave. Roadside bombs are a constant threat. Even Hassan, the dam manager, wants the Americans to go.

Hassan was at the dam last year when U.S. bombs destroyed the Iraqi army tanks and antiaircraft units nearby and cut the transmission lines for the second time in 12 years. Army Rangers came by helicopter and Humvee early that morning. Hassan gathered his 25 engineers and technicians on the second floor and listened in fear as the U.S. soldiers advanced from the top, breaking and exploding doors as they came. When they reached his group, Hassan nervously stepped into the hall and explained, in proper English learned in his engineering studies, that they were civilians.

The Americans were cautious, suspecting the dam might be rigged with explosives, but Hassan had an urgent task. If the pumps were not restarted, the whole building would be flooded in four hours, he explained. For two sleepless days, a CIA operative watched every move made by Hassan and his engineers as they nursed emergency generators to operate the pumps, he said.

With Iraqi spare parts and American help, most of the transmission lines were repaired. A Colorado company won a $10 million contract to help replace a broken turbine blade, catch up on a decade of maintenance and install modern switches. But Hassan says he is still fed up with his uninvited guests and with the restrictions imposed on his 200 Iraqi employees.

"We need our freedom," he said. "We have been in this power station for 18 years, and now I cannot even walk around my own plant."

"I understand his point of view," said Winn. But, he said, "this is a sensitive infrastructure. Without the electricity this dam provides, the quality of life for Iraqis would be substantially reduced."

It is also, Winn admits, a point of control for the military, being located near two major highways. Not long ago, the Marines say, a raid on suspected insurgents turned up a map of the dam marked with the locations of the American and Azerbaijani sentries and even the commander's room. They stepped up patrols and continued their raids.

Two hours before dawn one day last month, a force of Marines gathered for a raid in the delicious nighttime cool on the top of the dam. A full moon worked alchemy on the Euphrates, turning the water to quicksilver. Downriver, Haditha glittered with lights left on all night, one benefit of being home to a hydroelectric plant.

An arms dealer was believed to be stashing weapons on a tiny island six miles south of the dam. The Marines were ready for the raid, ready to show the land-to-water assault skills that makes the corps distinct.

Nine Humvees, towing two Zodiac rubber boats, slipped through the darkened countryside. When they turned on a palm-lined path toward the river, a barking dog and two braying donkeys announced their arrival, but if there was anyone on the island, it was too late. Five of the vehicles lined the riverbank, cutting off escape routes. Two Cobra and Huey attack helicopters appeared, circling the small island.

Nothing went quite according to plan -- it never does, the Marines grunted. They struggled to carry the Zodiacs down to the water, slipping on dried palm fronds and the slick riverbank. Four men plunged into the water, cursing at the muck and rocks, to mount the heavy outboard motors. The 55-horsepower engines were cold and would not start. Once going, the craft struck rocks in the shallow river. Eventually the first boatload of Marines had to wade waist-deep to the island.

The island was fiercely defended by bugs and briar bushes. The Marines pushed through, guns at the ready, to a clearing. It had once been farmed, judging by the furrows. The soldiers poked through the underbrush. Nothing. They waded around the perimeter, startling crabs. They used metal detectors to search for a weapons cache. Nothing. The island was swarming only with ants, inch-long creatures that scurried up pants and soon had to be brushed off faces.

"Sometimes it's a dry hole, sometimes it's not," the commander of the raid, 1st Lt. Christopher Dellow, 26, of Binghamton, N.Y., said with a shrug.

Sgt. Erik Lowe, 23, from Lapeer, Mich., was glum at the prospect of paperwork. "I get to go back and fill out reports saying there are lots of ants here," he said.

Staff Sgt. Eric Hodge, 31, a veteran from Port Huron, Mich., was even more disgruntled. The launch and the withdrawal from the island were sloppy, he told the tired Marines at the end of the morning.

"I'm not going to chew you out," he promised, then proceeded to do just that. No Marine sergeant's lecture would be complete without some old standards: "You girls looked like a bunch of Girl Scouts out there today," he barked. There would be more drills on the rafts, he promised.

Dismissed, the weary Marines headed to their bunks. Hodge permitted himself a wry smile. They didn't do that badly, he confided.

In a cool dawn light, Marines in Zodiac boats approach an island downriver from the Haditha Dam, on the Euphrates River in northwestern Iraq.Fearing sabotage, Army Rangers seized Haditha Dam in April 2003. U.S. and Azerbaijani troops have guarded it since.