“I’m dead.” The thought settled on Brad Snyder in the middle of a cloud of dust and smoke raised by the blast of a homemade land mine. He lay in the fetal position on a patch of grass next to a ravine in southern Afghanistan. The Navy lieutenant couldn’t make out any blood through the haze. His arms and legs were still attached. He didn’t know anyone who’d survived one of these explosions with all of their limbs intact.