(FORTUNE Magazine) - Going to war wasn't a textbook career move. In 2002, Jim Vesterman, an Amherst grad who had spent several years in the business world, was scheduled to begin the MBA program at Wharton. For some time, however, he had considered joining the military - both out of a sense duty and because he felt that it would challenge him like nothing in the civilian world could. Then 31, he wondered if this was his last shot.

Wharton let him defer admission, so Vesterman set his sights on the elite Marine Corps' Special Ops unit called Force Recon, whose members are drawn from enlisted men rather than the officer ranks. He was able to sign on for two years of active duty and two years in the Reserves, rather than the normal four to six years of active duty.

The catch: If he didn't make it into Recon, he was headed for a long tour, starting as a private. Vesterman knew that he'd learn to jump from airplanes and survive behind enemy lines. But he had no idea he'd learn so much about what it meant to be part of a team. He agreed to share his experiences - from his first day at boot camp through his tour in Iraq - with FORTUNE.

"I can't breathe! I can't breathe!" yelled a fellow Marine Corps recruit during our early boot camp training in the tear-gas chamber. The drill instructor's response: "I! I! I! What does 'I' have to do with anything?"

From the first day in boot camp, we were told we weren't allowed to use the word "I."

This was completely different from my experience in the business world, where I'd spent the past seven years. Throughout my career "I" had to do with almost everything. Though I considered myself a pretty good team player, personal success and achievement were my real benchmarks. Yet everything I thought I knew about working with other people was about to change. It started on April 22, 2002, the first day of boot camp on Parris Island--when we learned to make our beds.

It's called "two sheets and a blanket." When the drill instructor begins counting, you've got three minutes to make the bed - hospital corners and the proverbial quarter bounce. When you're done, you're told to get back in a line. The goal is to have every bed in the platoon made. So I made my bed, then I stood on the line. I was pretty proud, because when three minutes were up, there weren't more than ten men who had finished. "Ahead of the pack," I thought. But the drill instructors weren't congratulating us. Everyone's bed had to be made. So rip off the sheets and do it again.

I ripped off the sheets, made the bed, and stood on the line. "We've got all day to get this right," the drill instructors were saying, looking at all the unfinished beds. "Two sheets and a blanket!"

I ripped off the sheets again, and again, and again. Finally one of the drill instructors looked me in the eye. "Your bunkmate isn't done. What are you doing?" I thought, "What am I doing?" Standing on line, thinking I'd accomplished something, while my bunkmate struggled.

Together my bunkmate and I made our beds about twice as fast as we did alone. Still, not everyone was finishing. Finally we realized, "Okay, when we're done, we've got to go help the bed next to us, and the bed down from that," and so on. I went from thinking, "I'll hand my bunkmate a pillow, but I'm not going to make the bed for him" to making beds for anyone who needed help. That first lesson was an epiphany for me: You can't survive in the Marine Corps without helping the guy next to you.

After 13 weeks of boot camp, Vesterman graduated first in his platoon of about 50 recruits. He then spent eight weeks at Infantry School, where he passed a Marine Special Ops physical qualifying test called the Recon Indoc, and was sent to RIP (Reconnaissance Indoctrination Platoon), then to Amphibious Reconnaissance School (Recon School), considered one of the most challenging schools in the U.S. military. On average, 40% of those who have already passed all previous screenings will quit Recon School within the first week.

IN Recon school, everything gets more intense. Recon Marines are trained to conduct missions behind enemy lines in small teams, usually comprising six men. As the physical and mental challenges get more extreme, you become ever more reliant on your team - both to accomplish missions and simply to survive. We'd go for six or seven days without sleep, and in the middle of some chaotic exercise, the instructor would say, "The guy next to you is down. You get him to that ravine." I'm 5-foot-7, and it took more than I thought I had to put some 230-pound guy on top of the pack on my back and run. Until Recon School, I don't think I ever really let anyone carry my weight. I don't think I was man enough to say, "Hey, I can't do this" or "You're stronger than I am at this. I need help." Then came the telephone pole.

Running five miles while carrying a telephone pole is a grueling task - made more so when it's a race between your six-man team and several others. The only instructions: move our telephone pole along the route, don't let it touch the ground, come in first. "It pays to be a winner," shouted the instructor, a phrase in Recon-speak that let us know there was a reward for coming in first.

The race began immediately, so we had to come up with the best technique on the fly. There was no time to discuss a strategy or organize our process. My team struggled at first, but our approach evolved quickly. We had four guys carrying the pole and two resting by jogging alongside. When we switched off, we decided, it should be the two guys hurting the most - not necessarily the two who had been carrying the longest - who got to rest.

Nobody, especially a type A Recon Marine, wants to be the person who's "not carrying his weight." But some of these guys were simply workhorses - they could run forever with this thing - and some of us could not. We were learning to put team success ahead of our own egos. That was the only way we could move the fastest as a team and win the race. This was a huge change.

Vesterman was the Honor Grad in Recon School - graduating first in his class of around 50 Marines. He went on to complete Airborne Jump School, Marine Combat Dive School, and finally SERE School (Search, Evasion, Resistance, and Escape), where Marines learn to survive as POWs. He was moved to reserve status and two weeks later, in August 2003, began his MBA courses at the Wharton School.

Back in Philadelphia, I quickly became enmeshed in Wharton's own team structure, including my first-year 60-student MBA cohort and my learning team of six peers. In your first year, the majority of projects are done in these peer groups. You're thrown together, graded together, and have to keep up with a flood of assignments, so it's an intense experience. I felt I had made a strong commitment to this new team.

At the end of my first semester, I learned that my unit, 3rd Force Reconnaissance Company, was being called up to serve in Iraq. That same week Peter Pace, vice chairman of the Joint Chiefs (and now chairman), came to Wharton and spoke to my class. When it came around to question time, I raised my hand. "Sir, it looks like I'm being called to serve in Iraq. Do you have any advice for me?"

At that point I hadn't even told my classmates yet, and I had very mixed feelings about going to combat and leaving my Wharton team midyear. General Pace, a Marine, didn't reply for three or four seconds. Then he looked at me and the first thing he said was simply, "Congratulations." Immediately, everything became clear to me. The Marine Corps' highest purpose is to serve the nation in time of war, and it was my once-in-a-lifetime opportunity to serve.

The Marine Corps and the Army had entered Iraq in March 2003. Once a war is over it falls to the Army to keep the peace, but when the insurgency surfaced, and then continued, the Marines were sent back in. After several weeks at Camp Pendleton, Calif., in February 2004, Vesterman and his platoon were among the first Marines to be redeployed to the Anbar province in western Iraq.

The youngest guy on our six-man team was 24; the oldest was 38. Our backgrounds ranged from a plant manager to a SWAT cop. Some of us had known each other for a few months, others for nearly a decade. Six totally different lives. But very quickly it felt like we were moving as one - instinctively. We didn't do a lot of speaking. When you need something like binoculars from your pack, you don't stop, put the pack down, and open it, because if you're attacked, the group can't respond immediately. You just say to the guy behind you "I need binos," and you hold security for both of you while he gets them and reseals your pack. When he hits your pack, it means "ready."

There's a tremendous level of trust required between Marines. But since boot camp it's been drilled into all of us to do the right thing. Whether it's good for you or not. Whether it's easy or hard. Whether someone is watching or you're alone.

Our specialty was to enter a town unseen and set up shop there for days at a time in order to do reconnaissance and launch raids. Everyone on a Recon team has a different role, but each person needs to be able to move into any job. I learned in the Marine Corps that the strongest teams have members who transition easily between roles.

On one mission in a violent town along the border with Syria, we took up a clandestine position in an abandoned building that provided a good view of an intersection where many roadside bombs had been detonated. Our surveillance began to focus on a group of men who were meeting repeatedly on a nearby corner. The man who appeared to be the leader was wearing purple pants, so that's what we called him.

Suddenly we spotted these men retrieving objects in burlap bags from a nearby ravine and moving them in a pushcart toward the corner. They needed to move only 200 meters before they were out of our sights. While we thought the objects were bombs, we weren't sure, so we couldn't engage from our sniper position.

We had only seconds to confer. Then our team leader made the call: "Four men go and intercept." The other two would stay behind and maintain watch from an elevated and defensible position. What followed was the sort of chaotic scene our team was trained for. We exited our position and covered 500 meters at a dead sprint. A local lookout had warned the men, however, and as we turned the corner, the only thing visible was the pushcart.

We instinctively split into buddy teams - you never work alone - and my buddy and I began searching the area while the other team moved toward the pushcart. We recognized one of the men moving away from the scene and intercepted him. While I flex-cuffed the man, my partner moved to hold security.

The same coordination was happening with the other buddy team: One held security while the other opened the abandoned pushcart. As we suspected, there were two Saddam-era artillery shells inside, rigged as bombs. We immediately called this back to our team leader, who was then able to take a shot on "purple pants" when he momentarily reappeared at the original corner. "Purple pants" ended up escaping that day with only a shot to the arm, but we captured him a few weeks later.

We didn't react as six individuals that day. We were one team. Our platoon commander would often quote Kipling to describe the Marines: "The strength of the pack is the wolf, and the strength of the wolf is the pack." The Marine Corps recruits wolves. But its strength comes from training them to fight as a pack.

For his actions that day, Vesterman was awarded the Navy and Marine Corps Achievement Medal. After finishing a seven-month tour, he and his team turned in their gear. Six weeks later, in November 2004, he returned to Wharton.

Elasticity of demand. That's what they talked about in my first class back at Wharton. That culture shock was more jarring than going to Iraq. I slowly came to. In many ways there's probably no better preparation I could have had for the business world than joining the Marine Corps. The Marines teach you how to be both a leader and a follower. I don't have to lead in every situation - but I've come to enjoy stepping up in a time of chaos. When I'm working with a group now, I can honestly say that I think about the team first. The "I first" approach has been drilled out of me.

I'm certainly not the only veteran at Wharton, and for the past several years, the veterans' club has organized a field trip to the Marine Corps Officer Candidate School in Quantico, Va. Students with no military experience are suddenly crawling through mud and under barbed wire while drill instructors shout orders. Two months before I graduated, we made the trip again.

The first drill was familiar: two sheets and a blanket.