Sgt. John Newport left Iraq months ago, but he is still struggling with what he experienced there. He keeps playing one scene over and over again in his mind.
It took place when he was in his Humvee, passing a convoy of trucks that was hauling tanks. One of the truck drivers tossed a bag of M&Ms at a bunch of Iraqi kids.
A little girl went to pick it up, but there was a truck behind her. The driver didn't see her, and ran her over.
"The hardest part for me is that she was about the same age as my daughter is," Newport told "Nightline," with tears welling up in his eyes. "After that truck had run her over, you couldn't even tell it was a person."
Newport has what doctors call post-traumatic stress disorder, a condition often linked to depression and floating anxiety.
There are concerns that the Iraq war is producing more cases of PTSD than any conflict in decades because the violence has been so widespread and exposure to it so constant over long periods.
Newport says he began reliving the scene when he returned home to the Army base at Ft. Polk, La., and saw his daughter for the first time. "But it was no longer that little Iraqi girl," he said. "That was my daughter going into that road."
A study published in the New England Journal of Medicine found that one out of six soldiers surveyed may be struggling with PTSD.
More than 300,000 U.S. soldiers have served in Iraq. There are about 140,000 there now.
But the study was done early in the conflict. In many respects things have gotten worse.
"If there's increased threat, if they're more exposed, then there'll be higher rates," said Robert Ursano, director of the Center for Traumatic Stress and chief psychiatrist at the military's own medical school.
Iraq -- with suicide bombers, roadside mines and the constant threat of attack -- poses a unique challenge to the mental health of American soldiers.
"They're more at risk and they feel that risk in an ongoing way," Ursano said. "There's not an area of safety. It's all relative safety. In other wars, one has been able to have spots where, in fact, safety could be relatively ensured. That's less true in Iraq."
That extreme uncertainty is likely to cause even deeper psychological scars.
Harriet Barton and her husband, Michael, both Iraq veterans, also struggle with PTSD.
In Iraq, Harriet's vehicle ran over a mine. She took shrapnel in her leg and hand. Insurgents then attacked with mortars. For her, loud noises, like thunderstorms, have become a crippling trigger.
She remembers one night being so affected that she "felt like I was paralyzed laying in my bed and it was like I was reliving that entire situation all over again."
Michael says the sight of his wife in terror made him feel helpless. "To see your wife laying on a bed, grabbing her ears and basically screaming out to make it stop or something like that, it does something to you."
Harriet says she never thought her experiences in Iraq would haunt her like this. "I never dreamed that it would be anything like it is now," she said.
The Army has launched the most aggressive campaign in history to deal with these hidden scars. It offers help while the soldier is still in Iraq and when they come home, intensive screening and lots of counseling are provided.
But there is a huge problem: Soldiers often resist getting the psychological help they need because of the stigma.
Harriet says she knew early on "that something was wrong with me and I didn't want to admit that there was something wrong with me and I needed to get help."
Now, Harriet and her husband are in a group at Ft. Polk where all participants say they need help in coping with the haunting memories of Iraq.
Yet both she and her husband feel that they are not the only ones struggling with PTSD, and that there are far more suffering soldiers out there.