Shaded by a towering blue spruce in Wheeler Park stands a gray granite monument that honors this city's men and women who have died in combat from the Spanish-American War to, as the memorial reads, "Iraqi Freedom."

The name of Lance Cpl. Marty G. Mortenson was etched into the stone on the eve of Armed Forces Day in May. A month earlier, on April 20, Mortenson had been killed by a roadside bomb in Iraq.

Just a few months before he died, Mortenson sent his mother an e-mail: I am really sorry about [forgetting] your birthday . . . I am so streesed out that it is really bring[ing] me down. . . . I have had so much on my mind . . . going off to war 4 the 3rd time isn't easy.

Mortenson was on his third tour -- his third pump, in Marine jargon -- in Iraq. He had spent his 20th, 21st and 22nd birthdays in Iraq. Before he left on his last tour, he told a friend in California: "It's like three strikes, you're out. I have a feeling I'm not going to come home."

A generation ago in the Vietnam War, grunts had to survive 13 months and then knew they were going home for good. But the nature of an all-volunteer military has changed deployments and expectations for America's troops.

With the military's numbers at their lowest level in modern history, no draft to bring in new recruits and no end in sight to the U.S. deployment in Iraq, more American troops are likely to be going on multiple tours. The Army has sent multiple units to Iraq for second tours. The Marines, which deploy units for shorter stints, are embarking on third tours. Three infantry battalions and three rotary wing squadrons of Marines are on their third pump in Iraq.

At least 13 troops on their third tours, most of them Marines, have been killed.

"We're not expanding numbers, and we're not reducing our commitments around the world," said University of North Carolina history professor Richard H. Kohn, a former chief of Air Force history at the Pentagon. "We're taking it out of the hide, as they say in the military."

"If they have to go back a second or third time, particularly a third time, is it really fair?" he said. "I would call that an extraordinary burden."

Approximately 17,000 of the nation's 191,000 Marines are stationed in Iraq. "We certainly understand the individual sacrifice to go over three times, but seven-month rotations ensure the right mix of people go over, and it keeps the deployment cycle down to a manageable rate," said Maj. Jason Johnston, a Marine Corps spokesman. "As time goes on, we will see more and more of this."

For Mortenson and the members of the 1st (infantry) Battalion, 5th Marine Regiment, 1st Marine Division at Camp Pendleton, Calif., the orders for Iraq came in January 2003, in March 2004 and in February of this year.

"I remember before we went in, nobody's ever been in combat and we didn't know exactly what to expect. But we were all motivated that we could do it. We were really eager to go," said Lance Cpl. Eric J. Young, 22. Like Mortenson, he was a squad automatic weapon (SAW) gunner in Alpha Company.

A year later, the second tour was greeted with a certain amount of confidence, he said. "We had an idea of what to expect this time: the heat, everything bad about Iraq."

But learning about a third tour was tough.

"Those of us who had gone through [the first and second deployments] were pretty convinced we weren't going to go back," said Young, whose enlistment ends in August. "Honestly, I was kind of pissed off about going back."

Cpl. Matt Buchanan, 22, a machine-gun squad leader in Charlie Company of the 1st Battalion, 5th Marines, agreed. He was shot in the arm during his third tour this spring and was sent back early to wait out his September separation from the service.

"I thought, 'You have got to be kidding me.' I could not believe they were doing that," Buchanan said. "I can accept that because it comes with the job. But the worst part was telling my family."

Mortenson enlisted in May 2002 and was in the first large U.S. ground force to go into Iraq on the first night of the war in March 2003. The battalion secured a facility in the Rumaila oil fields along the Iraq-Kuwait border and pushed more than 300 miles north to Baghdad. Before returning stateside in August, they had engaged in fierce fighting with the Fedayeen and had taken over a mosque where Saddam Hussein had been sighted.

By fall 2003, Mortenson knew his battalion was headed overseas again, this time to Okinawa by mid-December and perhaps on to Iraq. The training cycle in Okinawa, however, was curtailed by February 2004.

"We were discouraged that he was going to miss another Christmas, and then he was only in Okinawa a month and he called and said, 'They're moving up Iraq,' " said Mortenson's mother, Ruth.

By March 2004, Mortenson and Alpha Company were in Fallujah. They commandeered an old potato chip factory they nicknamed FOB (Forward Operating Base) Wounded Knee and ran security patrols out of it.

It was during that first battle for Fallujah in April 2004 that Mortenson bailed out his entire platoon. A SAW gunner, Mortenson sprinted more than 300 feet under intense enemy fire to set up his machine gun to provide cover fire for his unit. He earned the nickname "Mad Dog" and was awarded the Navy and Marine Corps Commendation Medal for Valor in Combat.

Mortenson was back at Camp Pendleton in late summer 2004. He told a National Public Radio reporter who was interviewing Marines upon returning from their second tour: "I've been there twice, and no, I don't want to go back."

His mother recalled what he told her: "I've had it."

But by Christmas, Alpha Company knew it was headed back to Iraq. "You're thinking, we've had two pretty rough times. Is he going to have to go back?" Ruth Mortenson recalled. "And he said 'yes.' "

Mortenson spent his pre-deployment time off at home in January, snowboarding the mountains outside Flagstaff in a T-shirt. He wanted to soak up all the cold he could before going back to Iraq, his family said. He baby-sat his nephews and built them forts in their living room. He took his "dream girl," a former classmate he knew would never agree to a real date with him, to lunch. He said he wanted a grilled steak, so his father, Ken, put up a tarp in the back yard and shoveled the snow out of the barbecue pit.

"We realized we had to make the most we could of when he was home," Ruth Mortenson said.

By March, Mortenson and the rest of his unit were in Ramadi -- just kind of doing a lot of patrols looking for the enemies, and guarding iraqi political centers, Mortenson wrote his parents on March 13.

He asked his parents to send phone cards and batteries, and he tried to calm their fears.

the ieds or . . . road side bombs, they happen all the time and arent very effective, the enemey only uses them because they can detonate them from far away from the safety of us . . . i am fine dont worry, Mortenson wrote on March 27.

In Flagstaff, the Mortensons immersed themselves in their work and in their prayer and Bible study groups. "We went on with our lives, trusting we would hear from him eventually and he would be all right," his mother said.

In April, Mortenson began writing home about life after the Marines, and by the middle of the month he knew his third tour was to end in mid-October. He wrote that he was interested in joining the National Guard, working as a firefighter or attending community college to learn auto body work.

i am trying to put out ideas because on may 19th I only have a year left. that only leaves me with 6-7 months when I get back . . . not a lot of time, Mortenson wrote on April 18.

Early on the afternoon of April 20, Mortenson and Cpl. Kelly M. Cannan, another third-timer in Iraq, were on their way to catch reported terrorists at a cafe in Ramadi when a roadside bomb went off beside their Humvee. Cannan was killed instantly. Mortenson sustained a massive head wound and died hours later at a military medical facility in Baghdad.

In Arizona, two Marines arrived early on the evening of April 20 at the Mortensons' blue-and-white tract home in Flagstaff. They asked for Ruth, the beneficiary listed on her son's $250,000 military life insurance policy.

Five hundred mourners packed Flagstaff Christian Fellowship for Mortenson's funeral on April 27. The family's scrapbook is six inches thick, with hundreds of sympathy cards and e-mails from friends and public officials such as Flagstaff's mayor and City Council members, President Bush, and California Gov. Arnold Schwarzenegger.

Ruth Mortenson recalled those days before Christmas when she first heard that her son might have to go back a third time and her worries were renewed.

"If they go back, can they put him someplace easy?" she remembered thinking. "But Marines don't go to easy places."

Researcher Madonna Lebling contributed to this report.

Lance Cpl. Marty G. Mortenson of Arizona was killed on his third tour.One week after their son was killed, Ken and Ruth Mortenson sat at his graveside service. "I've been there twice, and no, I don't want to go back," Marty Mortenson had told an interviewer before being deployed again.A Marine honor guard carried the casket at Marty Mortenson's funeral, which was attended by more than 500 people in Flagstaff, Ariz.