If slavery was truly the United States’ original sin, then Vietnam is our guilty conscience. It was too complex and too disorienting and too many Americans lost far too much for the nation yet to understand it fully. More than 30 years later, the country still labors under the psychological weight of that experience. They say that the victors write history, but this was a war that nobody won. The South Asian battlefields may have long fallen quiet and the domestic tumult that pit parents against children and brothers against brothers may be over, but as long as many of...