In January 2004, six months after leaving Iraq behind, Dwyer started having flashbacks. They woke him from sleep in a cold sweat. Even in the light of day his mind returned to the battlefields—the smoky stench of spent ammo, the rattle of the radio, the sight of bloated bodies. For a while he was nothing more than an unfortunate statistic, one of the thousands of Iraq-war veterans diagnosed with post-traumatic stress disorder.

According to a 2008 RAND Corporation study, 18.5 percent of Iraq and Afghanistan veterans suffer from PTSD or major depression and 19.5 percent report a "probable traumatic brain injury," which can lead to other mental-health issues. That amounts to more than 300,000 vets already—before the surge returns from Iraq. And a great many of those soldiers have gotten inadequate treatment or no treatment at all. "We were trained in how to deal with a sucking chest wound," says Adam Beard, a former cavalry captain. "But we weren't trained for how to spot a mental-health problem. We didn't really talk about it."

A year after his return, Dwyer thought he'd found some peace. Never particularly religious, he happened on a prayer breakfast at the base and was struck by a Bible passage, 2 Corinthians 5:17: "If anyone belongs to Christ, there is a new creation. The old things have gone; everything is made new." Raised Roman Catholic, Dwyer started attending a Southern Baptist church with his wife, Matina, taking the verse's message to heart. He was ready to be saved. On a Sunday morning in December 2004, he followed his pastor into a shallow wading pool, where the pastor dipped him backward until the water ran over Dwyer's head. He cried, praying for the promised cleansing of his soul.

He was granted no such miracle. The flashbacks turned darker and came more frequently, and with them the doubts. Not even God can save me. Dwyer stopped going to church and was consumed by thoughts of his certain damnation. Nothing made the anguish go away—not time, or the therapist he saw every six weeks, or the antidepressants he'd started taking. So he got something that did. One afternoon, when Matina was at work, he reached for a can of Dust-Off.

He put the nozzle to his mouth. He push the trigger and inhaled deeply.As he unleashed a chilly stream of vapor, a pressurized pffffffffft, the compressed air—used to clean computer keyboards—filled his lungs, limiting the oxygen to his brain. And suddenly Dwyer wasn't at war anymore. He felt giddy and light-headed . . . at peace. He knew huffing was dangerous: Within seconds of even the first hit, Dust-Off can cause cardiac arrest. Which is why, at first, the soldier kept his habit to a minimum. One can lasted days, letting him sleep and function normally. Soon, though, he needed 10 cans a day, chasing the 20-minute high with hit after hit, often late into the night.