For US veterans, what does it mean to heal a moral injury?

The Marine Corps observes its birthday every Nov. 10. On that day in 2004, as the Corps turned 229, Peter Giannopoulos spotted his friend Ryan Berg inside a mess tent on a U.S. military base in central Iraq.

Both men had deployed weeks earlier with the Marine Corps Reserve to an area known as the Triangle of Death. By then, less than two years after American forces had invaded, bloodshed and chaos engulfed the country as the Iraqi insurgency burned to full flame.

The two corporals smiled at the chance meeting, their first in the war zone. “Happy birthday, Marines,” Giannopoulos said to Berg and a few others seated by him. Berg chatted with him for a minute before Gino, as most called him, strode outside to join his platoon for its next mission.

The following day, after returning to base from a patrol, Berg stood talking with his squad leader. Another Marine from Giannopoulos’ unit rushed toward them. Distress choked his voice. “It’s Gino! It’s Gino!” he said.

Insurgents had attacked their platoon. Gino was 22. He died on Veterans Day.

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The loss of Corporal Giannopoulos on a distant battlefield 15 years ago lies at the heart of Mr. Berg’s complex emotions about the Iraq War.

“When you’re there, everything has extreme significance. It all really matters in the moment,” he says. The demands of combat left little time to ponder the reasons for the U.S. military’s broader mission or to grieve for Gino. “You’re trying to survive. That’s it.”

The passage of years has brought the fog of war into focus for Mr. Berg, who deployed again to the country in 2006 and whose doubts  about the conflict’s purpose have sharpened. He regards the ongoing turmoil in Iraq, where security forces in Baghdad killed more than 100 people

The toll of moral injuryA deepening regretMending the spiritSearching for closureA devotion to fellow veterans