It was the winter of 2013, and after three tours in Iraq and four in Afghanistan, Mr. Merkle had spent years struggling with the invasive symptoms of post-traumatic stress disorder. He felt constantly on edge, bracing for an attack. He got angry easily. He avoided thinking or talking about his time as a Marine; he tried traditional talk therapy, but didn't feel ready to discuss his past.

Months later, after his symptoms intensified and he felt desperate for a salve, he decided to give virtual reality exposure therapy a try at a Department of Veterans Affairs hospital in Long Beach, Calif. The treatment uses V.R. technology to immerse a patient in a three-dimensional environment that mimics a traumatic memory. He strapped into a headset and sank into the past.

The details in the simulation were extremely precise, Mr. Merkle said: the military-issue truck, the weight of the model gun in his hand, the dark swath of sand in the night. He narrated one particularly troubling incident out loud to a clinician, who adjusted the simulation as he spoke. "I was seeing that person shooting at me, that I hadn't thought about in 10-plus years," he said. His muscles tensed. His heart raced. He was terrified.

"My body was physically reacting, because my mind was saying, this is happening to us." But when he took the goggles off, he said, the sense of accomplishment became its own form of comfort. For years, his memories had terrified him. Confronting the past in V.R. proved to him that he could survive revisiting his memories. "That was the biggest leap," he said.