He enrolled in college and "tried to get into nonveteran things: student government, honor society, business society. I wanted to broaden my identity." Too many veterans cling to their military status "like a guy holding on to an old girlfriend who really doesn't want you anymore," he said.

While at San Diego State, he helped found the Student Veterans Organization, not to swap war stories or complain that civilians don't understand them but rather to help veterans navigate the sometimes byzantine world of financial aid and graduation requirements.

In 2005, as the war flared anew in Iraq, Donnelly considered returning to the military as an officer. He was blocked by a medical condition; looking back, he is not upset about the rejection.

He is now pursuing a masters in business administration and hopes to land a job in international development. Military service does offer a unique camaraderie, he said, but it also can limit someone's ability to see other ways of thought or to relate to people who have never worn a uniform or fired a rifle.

"I was proud of my service, but I knew I had to get back to regular American society," he said.

For many soldiers, the scars of war will persist and their thoughts will linger on the most intense experience of their lives. Many also will carry a renewed appreciation of the precious commodity that is life and peace.

Robert Parry, 39, a marketing director for a Newport Beach investment firm, deployed with the California Army National Guard to Kuwait after the 9/11 attacks and later served in combat in Iraq and Afghanistan. In Iraq, Parry was with a Guard unit that lost 17 soldiers in action.

Now a captain, he said leading soldiers in battle brought a sense of fulfillment and achievement. "It's a truth that civilians cannot understand," he said. "They talk as if deployment is a bad thing."

But after returning to USC to pursue his MBA, he felt like "a heart-attack survivor" — as if he had won a second start on life.

"The trees seem greener, the sky more blue," Parry said. "Things that used to bother me didn't matter as much."

Full coverage: A decade after 9/11