Over three million young men left home, shouldered rifles, and set about killing one another in the 1860s. Behind, they left wives and sweethearts. The 50,000 books about the war have told us in meticulous detail about the strategy, tactics, weapons, uniforms, canteens, famous generals, religious beliefs, personality quirks, fortifications, battles, sieges, gunboats, medical care, and recruiting policies. The causes of the war have been endlessly analyzed. The surviving veterans wrote hundreds of memoirs, sometimes inflating their own heroism and importance. What rarely appears in this...