Soldiers returning from Iraq and Afghanistan are a generation of warriors whose fight has shifted from external combat zones to invisible internal battlefields.

At 8:20 p.m. on Sept. 21, 2010, Iraq veteran Brock Savelkoul decided it was time to die. He lurched from his black Tacoma pickup truck, gripping a 9-mm pistol. In front of him, a half dozen law enforcement officers crouched behind patrol cars with their weapons drawn. They had surrounded him on a muddy red road after an hour-long chase that reached speeds of 105 miles per hour. Savelkoul stared at the ring of men and women before ducking into the cab of his truck. He cranked up the radio. A country song about whiskey and cigarettes wafted out across an endless sprawl of North Dakota farmland, stubbled from the recent harvest. Sleet was falling, chilling the air. Savelkoul, 29, walked slowly toward the officers. He gestured wildly with his gun. “Go ahead, shoot me! … Please, shoot me,” he yelled, his face illuminated in a chiaroscuro of blazing spotlights and the deepening darkness. “Do it. Pull it. Do I have to point my gun at you to … do it?”

Twenty feet away, the officers shifted nervously. Some placed their fingers on the triggers of their shotguns and took aim at Savelkoul’s chest. They were exhausted, on edge after the chase and long standoff. They knew only the sketchiest of details about the man in front of them, his blond hair short, his face twisted in grief and anger. Dispatchers had told them that Savelkoul had been diagnosed with post-traumatic stress disorder. They warned that he might have been drinking. Family members told police that Savelkoul had fled his home with six weapons, including a semiautomatic assault rifle and several hundred rounds of hollow point ammunition. To Megan Christopher, a trooper with the North Dakota Highway Patrol, Savelkoul’s intentions seemed obvious. “Suicide by cop,” she thought. “He wants to go out in a blaze of glory.”

As it happened, Savelkoul’s state of mind was of interest not only to the cops, but to some of the nation’s top military officers and medical researchers.

More than 2 million troops have deployed to Iraq and Afghanistan since 2001. Tens of thousands have returned with a bedeviling mix of psychological and cognitive problems. For decades, doctors have recognized that soldiers can suffer lasting wounds from the sheer terror of combat, a condition referred to today as post-traumatic stress disorder. They also have come to know that blows to the head from roadside bombs — the signature weapon in Iraq and Afghanistan — can result in mild traumatic injuries to the brain, or concussions, that can leave soldiers unable to remember, to follow orders, to think normally. […]