Lucas Wallek was toiling at the family farm in Alice, Texas, when he decided to enlist in the Marine Corps, partly out of patriotism, partly from a desire to leave the familiar terrain of sagebrush and oil fields.

Through two tours of duty in Iraq, Wallek, 27, said he found a sense of purpose and camaraderie he had never experienced. His friends back home, he said, "don't see the world the same way I do…. Sometimes it's hard to be with them; it's like I miss my Marines. We see the world the same way."

Nathaniel Donnelly, who was part of the Marine push in Iraq that helped topple Saddam Hussein in the spring of 2003, said his time at war left him proud, yet also weary and afraid that he was being drawn further away from the civilian life he wanted one day.

Full coverage: A decade after 9/11

"Military service sets you apart — whether that's positive or negative is up to the individual," said Donnelly, 34. "With a lot of veterans, they seem to have a chip or really can't relate to the civilian community. I see it all the time. Some guys just can't get back on track."

Wallek and Donnelly are among the men and women who in the wake of Sept. 11 have spent much of their adult lives at war.

Their worlds have been shaped by training and deploying, grieving the loss of comrades and recuperating from injuries, learning new techniques for killing and for staying alive.

Like the decade-long conflict in Vietnam, the wars in Iraq and Afghanistan have persisted so long that, in a sense, they have become a way of life.

And while many say being in the military has given them a sense of self, there is a nagging regret over the youth that they missed.

At a formative time in their lives, young volunteers enter a world of discipline and orderliness, leaving behind community, family and potentially a wider sense of life's possibilities. "It's a trade-off," Donnelly said.

The demands of war have taken their toll — on the country, the military and families.

There have been more U.S. combat fatalities, more grievous wounds, more amputations, more cases of post-traumatic stress disorder than were imagined when President George W. Bush launched the war on terror.

More than 6,200 uniformed Americans have returned home in flag-draped coffins to the mortuary at Dover Air Force Base in Delaware; nearly 46,000 have received Purple Hearts.

For those who have made it home, suicide and divorce rates have been higher as veterans have struggled to deal with the memories of war and the pressures of civilian life.

More than 200,000 personnel remain deployed in or near the war zone. And even with a planned U.S. drawdown, the Marines have been told to expect deployments to Afghanistan through 2014, maybe longer.

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Wallek, a rangy, 6-foot-2 sergeant with a reputation in his unit for volunteering for hard duty, readily admits that when he signed up for the Marines, he was looking for revenge for 9/11.

He joined the Corps in 2005 and trained as a machine gunner. He chafed to get into the war.

"I saw the death toll rising in Iraq and I figured it was my turn to get over there and get some," Wallek said.