Alone and in clusters, collars up to block the rain, thousands of people lined the streets on a gray October day in 2005 to welcome their warriors home. For 13 miles, they rose to wave, a few to salute, as the buses rolled slowly past. More than one tough Marine, homeward bound after a brutal tour in Iraq, shed a tear. When they reached solid ground, still wearing their desert camouflage, the Marines embraced their families and embarked on the most jarring of transitions. They would discover in the following year that seven months in Iraq had changed them more than they could have imagined, guiding and afflicting them in ways they are still struggling to understand. Marines who expected duty so light that boredom seemed probable instead saw almost daily combat and 23 men killed in action, more casualties than any U.S. company in Iraq. When it was over, they traded an edgy, exhausting regimen of forced alertness and sudden brutality for sheer ordinariness. Nothing at home felt as urgent...