The Civil War is the greatest trauma ever experienced by the American nation, a four-year paroxysm of violence that left in its wake more than 600,000 dead, more than 2 million refugees, and the destruction (in modern dollars) of more than $700 billion in property. The war also sparked some of the most heroic moments in American history and enshrined a galaxy of American heroes. Above all, it permanently ended the practice of slavery and proved, in an age of resurgent monarchies, that a liberal democracy could survive the most frightful of challenges.

In Fateful Lightning,...