As commanding and revelatory as the recent best-sellers Flags of Our Fathers and Black Hawk Down, this new volume on the Vietnam War ranges from an obscure Cambodian island in Southeast Asia to the Oval Office of the White House as it chronicles one of the most overlooked incidents and heartbreaking episodes in America's costliest foreign conflict. On May 12, 1975, barely two weeks after U.S. helicopters lifted off the roof of the American embassy...