He has trained in the desert. In helicopters. On blown-out urban landscapes. Now Shannon Plaut is in a classroom, intent as a combat medic drills him and 47 other Marines on the effects of weapons almost too horrible to comprehend.

"Hey doc, what's that gas that makes you throw your friggin' guts up?" one of Plaut's classmates asks.

"VX," the medic tells them. Deadly. Invisible. Odorless. He asks if they can recall the nerve agent used in a 1995 terrorist attack on a Japanese subway.

"Sarin," several answer.

"Saddam Hussein also has large piles of sarin," he tells them.

These could be life-or-death details if Plaut is called upon to administer combat first aid -- that is, if there is a war, against an enemy capable of such destruction, which once seemed theoretical but now feels almost imminent. Plaut recites a grim fact of Marine life after class: "You have nine seconds to get your mask on."

Now there is Iraq.

Now, even for Plaut, who only nine months ago was graduating from the grinding misery of boot camp at Parris Island, S.C., Iraq looms as the mission that could thrust him into the real-world combat of enemy fire and urban warfare and chemical weapons.

It was not something he or others predicted when they enlisted in the weeks after Sept. 11, 2001. Back then, invading Iraq was not on the radar of daily public debate. And even as Afghanistan was pounded by bombings, ground troops were not deployed en masse.

But as Plaut's classmates of Sept. 11 Marines end a first year, they stand to see their military lives defined, a second time, by the turn of world events. Especially for many based here, at Camp Pendleton, any U.S. action in Iraq would mean a strong chance they will be in the Persian Gulf.

This has imbued life on this vast and rolling Marine base, set in the scrubby foothills of coastal mountains, with a kind of edgy uncertainty -- a reflection of the nation's sense of teetering on the verge of war, only sharper and more pointed.

Here are those who would help wage the fight, the same as a decade ago, when Marines from this base battled Iraq in Desert Storm.

Already there are signs of the combat to come. In early November,the headquarters staff of the 1st Marine Expeditionary Force, based here, moved out to Kuwait, in position to help coordinate any invasion of Iraq. Just before Thanksgiving, several hundred more planners and commanders followed. Thousands of other Marines await orders.

No one knows with certainty whether an invasion will happen, and much depends on the progress of the United Nations inspections that make headlines every day -- and which leave Marines such as Plaut and his roommate, Daniel Wilson, 22, studying the television news in their small barracks room with a new intensity.

Will they head out -- or won't they?

They know the call could come at any time. Posted in an office for their unit -- the highly decorated 2nd Battalion, 5th Regiment -- are maps of Iraq and Kuwait, with details about transportation routes and marsh destruction. Their routine now includes donning gas masks once a week. Shots are being administered for anthrax.

When Plaut was selected from his platoon to take a week-long course in combat first aid, which included the classroom discussion of nerve agents, he clearly understood where the next battlefield might lie. A rangy college graduate with dark eyes and prominent eyebrows, Plaut was upbeat but plainspoken about the grimness of it all.

"That's the only thing that bothers me if we have to go into a conflict," he said. "You don't smell or see the biological agents, and it's a lot harder to run and hide from gas."

Rough From the Start

One year ago, Shannon Plaut and 424 other men and women were enjoying the Christmas holidays with a sense of great foreboding. Within days, they knew, their lives would change in ways they could not possibly imagine.

They were joining the Marine Corps in the wake of Sept. 11 and had orders to show up for boot camp on Parris Island in the first days of January. They were nervous and scared, having heard stories about how demanding it would be, how utterly harsh.

A year later, as another Christmas passes, their lives have been remade, and yet there is a tinge -- some will admit -- of the same worry. The future looms large and unknowable. But now the concern is not change. It is danger.

"Deep down, I think everybody is always a little apprehensive," said Plaut, trained as a rifleman and a radio operator. Most Marines, he said, try not to dwell on the risks. As he put it: "This is what I came to do."

It is a long way to have progressed in a year. It was Jan. 2 when Plaut and many of his classmates took a midnight bus from a nearby airport to their fabled boot camp, on a swampy, flea-infested island on the South Carolina coast.

As they lined up in the darkness on yellow footprints painted on the street -- for their ritual introduction to life as a Marine -- a drill instructor thundered at them: "You WILL do what you're told to do, when you're told to do it, without question."

This became the theme of their lives. They gave up their clothes, hairstyles, phone calls, free time -- almost everything -- for 12 weeks of screaming-tough Marine training. They endured physical rigors like nothing they ever knew. They were taught to fire M-16s with killing proficiency. They made it through a 54-hour forced march called "the crucible."

When they graduated March 29, Good Friday, the intensity of training did not stop. For Plaut, it was six weeks at the school of infantry. For others it was a shorter course of Marine combat training and then weeks or months of special schooling.

Not everyone's path was the same. Some men and women dropped out, even after they made it through boot camp. Others were disciplined. A number were married or became parents. Some were disappointed by less-than-glamorous assignments.

By summer, many of the Sept. 11 Marines were standing in airports, carrying duffel bags and bound to their first duty stations -- in North Carolina, Hawaii, Okinawa, California -- with the guess that it might be many months or years before they would head off toward war.

But in August, Chris Funk and Michael Garey, both members of that post-Christmas boot camp class, got an early look at life in a hostile zone. As part of the war on terrorism, they were sent to Zamboanga, in the southern islands of the Philippines, where a Muslim terror group was planting bombs and taking hostages.

By autumn, many in the larger class of Sept. 11 Marines were wondering whether they might ultimately head toward combat and peril -- as part of an invasion of Iraq. In an age of talk about "scorched earth" retaliation and weapons of mass destruction, this had an ominous undertone.

Itching for the Fight

In the small ordnance unit in which John Adams works -- called "the junk shop" -- they are packed for Iraq. They have stowed away all sorts of parts for their trailers, which haul explosives that are loaded onto helicopters. If the trailers need fixing, they are ready.

Thinking ahead, they have also bundled in tents and gear and that old, most basic staple of combat -- rations. Tucked away and loaded, they have MREs, Meals Ready to Eat, shelf-stable dinner pouches with entrees such as bean burritos and chicken teriyaki.

Adams himself is a little wistful about all of this. He is not on the list for immediate deployment.

Steady in his manner, Adams, 19, from the Dundalk area of Baltimore, has had his eye on combat for a long time. While he joined a class of Marines who enlisted because of Sept. 11, he himself had planned to sign up anyway -- and he never wavered when terrorism changed U.S. war plans entirely.

"It just puts anger into you," said Adams, a graduate of Patapsco High School, where he sang in a range of school choirs and was captain of the wrestling team. "Although anger's not good, it kind of helps you get through."

Now, Staff Sgt. Vincent DiVincenzo, one of his superiors, mentions that Adams may not deploy for Iraq in the first wave of troops. He tries not to reveal his disappointment. His thinking has been: "I want to see as much action as I can."

DiVincenzo tells him: It could still happen. "The names are changing all the time."

Adams nods his head, heartened. They are standing in a fenced field that everyone calls "the bomb dump," where warheads are created for helicopters such as the Cobra, which congregate noisily on a nearby flight line.

This is where Adams hopes to land what he believes is a better ordnance job -- closer to the action. In the best of all cases, he thinks, he might do this in Iraq. "It's definitely dangerous," he said, "but it's something I want to do."

Running Into Trouble

In the rolling fields and cactus-studded hills of Camp Pendleton, not far from its 18 miles of Pacific coastline, Plaut's battalion spent three December days training in a city under fire, or at least the $10 million simulation of one -- 31 buildings set up like a town, built for this purpose, where Marines learn the art of clearing rooms and counter-sniping and urban patrol.

This is practice for what is called urban warfare, among the most difficult forms of action -- with too many hiding places for the enemy and too many civilians in the mix.

This is the kind of combat that could happen in Baghdad, a fact that is well-appreciated by those who could deploy.

Some are gung-ho about heading to the Middle East, and others, like Joey Sees, are a little more ambivalent.

"If it was up to me," said Sees, "I wouldn't go. But I'm ready to go. I believe the Marines have trained me for it. But who wants to go to war?"

Just barely over 18, Sees is one of the youngest Marines around, boyish-looking, with red cheeks and blue eyes, an enlistee on the day he turned 17 and one of the rare Marines without a high school diploma. From a tough Philadelphia neighborhood, Sees was raised by his mother, who managed a bar in nearby Camden, N.J. At 16, he dropped out of school and landed in the service pit at Jiffy Lube.

On Sept. 11, 2001, he was doing a shift as a waiter at a Denny's restaurant when a regular walked in screaming: "New York City's been attacked by terrorists!" The customers left. The employees watched television.

"I was enraged," Sees said. He had relatives in New York -- cousins and aunts and uncles. "I knew I had to do something. They came in our back yard and disrespected us. I guess that's the way I was brought up."

When Sees looks back at his first year, he thinks about infantry school, where hikes lasted days in woods overrun by ticks and chiggers -- no showers, no phone calls. "It's really physically hard," he said. "You have to be in tiptop shape, and you need to give 110 percent, even if it's just getting a haircut or cleaning your room."

Lately Sees has tried to avoid listening to the news about Iraq. "We've been told we were going a couple of times," he said. "I'm tired of being told we're going. We're going, we're not going -- everybody's talking, and we don't know for sure."

The brink of war, he has decided, is a fairly unsettling place. The thought of casualties has crossed his mind. "It's natural," he said, "to think of the worst and pray for the best."

His best friend here is Tim Kalla, whom he met at boot camp. Many other Marines have helped him out, he said. With a slight tug in his voice, he added: "We're all coming back together. There's no way that not every one of us is going to come back."

The greatest payoff for all the hard work, he said, has been going home in uniform. "There's not a lot of Marines where I come from," he said. "This gave me self-respect. I respect myself 150 percent more than I once did."

As Christmas neared, however, his future grew murkier. He found himself in trouble with the Marine Corps and designated UA -- unauthorized absence -- which is strictly forbidden. He would not talk about what happened. But his Marine life was ending. He was being let go.

By late December there was but one way Sees would remain a Marine well into the new year. The dismissal process typically takes four to six weeks. If the nation goes to war in that time, Sees will likely deploy to Iraq.

Waiting, Worrying

Thomas Butler and Gianna Wunderlich operate from a two-story barracks of classified offices, where Marines who work in intelligence at Camp Pendleton spend their days with bits of information about things like enemy tactics and weapons and battlefield terrain and transportation routes.

In collaboration with a small group of other intelligence Marines, the two newcomers help analyze what it all means. One week they are part of a simulated attack on Camp Pendleton, and the next week they are working up data on foreign lands.

Wunderlich, 20, now among the 5 percent of Marines who are women, enjoys the work. But when she first started working with real-world intelligence -- not just classroom scenarios -- she thought, "Oh my God, what am I doing?" In that moment, she realized the stakes.

"In school, if you screw it up you really don't hurt anyone," she said. "Here you're watching out for a whole division."

Butler, 21, said the job has changed his way of thinking -- now he examines everything with a sharper eye for detail and motive. He works as part of the same unit as Wunderlich, with sensitive information he is unable to mention outside his office.

His wife has stopped asking about his day.

One night, Butler comes home and falls asleep on a big blue recliner, as his 2-year-old daughter, McKinsey, and his boxer, Bridgette, play nearby. The television flashes with "The Little Drummer Boy." Holly, his wife, has made popcorn. The apartment twinkles with Christmas lights.

A year ago, Butler was working as a welder on construction sites in the Washington area, married just 15 months and the father of a baby girl. It was Sept. 11 that shifted his sense of the world.

That day, he was on the job in Virginia when he literally heard the hijacked plane crash into the Pentagon. Soon he was on the phone with his brother, who was at work 11 blocks from the World Trade Center in New York.

In the weeks that followed, he decided to enlist. His father had been a Marine. Now it was his turn.

All of this came as a surprise to his wife, with whom he had once discussed joining the military. "I was shocked," she said of the day he came home and told her he had talked to a recruiter on his lunch break. "We had made a decision that he wasn't going to do that."

Holly Butler said she could tell by his face that "this was something that he really wanted." But with war just starting in Afghanistan, she worried. "Are you going to get sent over?" she asked.

"I honestly don't know," he told her.

Now, in their small apartment, Holly Butler tries not to think of him heading toward Iraq. Several Marines in his unit have already gone. Beside his blue recliner is a wall adorned with Marine Corps photos and plaques.

They are hoping to move from their apartment to base housing soon, Holly said; she has concluded that the pay and housing allowance of a young Marine are not enough to afford off-base expenses and a car, too. They struggle to make ends meet.

"I worry, what's going to happen to him," she said. She braces herself: "At some point in time they are going to go. I know that it's just a matter of time."

Nonetheless, Holly planned a Christmas turkey and ham for the holiday, hoping her husband would still be on the continent. She placed gifts for him and her daughter under a small tree. "I'm trying not to think about it," she said, "until the time comes."

Love and Valor

Shannon Plaut is struggling up the side of a brush-covered hill, on a warm breezeless Wednesday, with another Marine on his back. He is sweating -- hot, drippy, red-in-the-face, hard-work sweat. His buddy is dead weight, uphill.

Plaut is breathing heavily, fighting gravity and weariness to get to the top. This, he knows, is how he would haul a wounded comrade on a battlefield. Not that he ever wants to see another Marine wounded. But this is for war. It could happen.

In the classroom afterward, Plaut sits with a cold bottle of Evian he bought at a convenience store along the way, not long before he walked into his classroom building and passed a poster on a wall that said: There are only two kinds of people on the battlefield. The hunters and the hunted.

Now the instructor is reviewing the day's material.

"Radiation sickness," the instructor booms. "What is it characterized by?"

The class knows: Nausea. Vomiting. Loss of appetite.

Then: "How about biological warfare?"

Before the Marines finish answering, he asks: "How many of you received your anthrax immunization today?"

Some 20 hands go up in a class of 48. Plaut takes it all in, and while most of what he hears is not new, it is presented in greater detail than before. In spite of this, Plaut remains cheerful, gung-ho.

He is here at 25, with a college degree, life experience and no shortage of forethought.

On Sept. 11, Plaut was working freelance as an actor in Florida and waiting tables to get by. His acting gigs were at places such as Disney World and Universal Studios, playing Beetle Bailey and the Cat-in-the-Hat, but his niche was stunt work.

That week, his father called Plaut from the Maryland suburbs -- he had grown up in Gaithersburg -- and asked: Have you given any thought to the military? Plaut had once wanted to enlist, and his father -- who served in Vietnam as an infantry adviser and Army officer -- had talked him into college instead.

Now, his father was suggesting the idea.

Plaut thought hard -- and jumped on it.

Before going to boot camp in the first days of January, he sold his furniture and his car in Florida and bid goodbye to his college sweetheart and longtime girlfriend, Jessica Hendrickson.

In spite of the separation, the relationship stayed strong, and on a Thanksgiving break at his sister's house in Denver, his girlfriend flew out -- and Plaut proposed to her in the airport.

They wed on Thursday, a date hastened by the possibility of war. The couple had been hoping for a spring wedding but feared Plaut would be in Iraq.

If that day does come, Plaut knows, his thoughts of home will include his father, Bill. In some ways, the terror and war of late have brought them closer. While he was at boot camp, Plaut and his father corresponded regularly. "It gave us something we didn't have," the elder Plaut said.

It was two days after his son graduated from Parris Island that Bill Plaut took him aside and placed an engraved medal in his hand.

It was a Bronze Star, awarded for combat valor in Vietnam. Looking at his son's eager young face, Bill Plaut told him quietly, "This is nothing you should aspire to. You have nothing to prove."

The 2nd Battalion, 5th Regiment spent three days in December learning to battle snipers and patrol cities -- tactics they could need in Baghdad. Shannon Plaut, 25, center, of Gaithersburg participates in a combat lifesaving course at Camp Pendleton, Calif. "I'm ready to go if I have to," Plaut says of the possibility of going to war in Iraq. "It's kind of why I came to the Marine Corps. If I'm afraid, I'm just going to bear with it and act like I'm not." Thomas Butler, a Washington area welder on Sept. 11, 2001, now lives with his wife, Holly, and daughter, McKinsey, in California. He is not allowed to talk about his classified intelligence work. The 425 Marines who began boot camp together in January have since scattered around the globe. At Camp Pendleton, Calif., some are training for a potential war in Iraq. Thousands of Camp Pendleton's Marines have shipped out for the Middle East. Those who remain prepare for war in an uncertain age.