Mr. Bordewich's most recent book is "America's Great Debate: Henry Clay, Stephen A. Douglas, and the Compromise That Preserved the Union."

One of the most fascinating passages in "Terrible Swift Sword" describes an encounter that took place not on a battlefield of the Civil War, where Union Gen. Philip Sheridan earned his fame, but in German-occupied France during the Franco-Prussian War of 1870-71. Sheridan, on leave from Indian-fighting on the western frontier, had attached himself as an observer to the victorious Prussian army, where he established a personal friendship with Otto von Bismarck, the architect of German unification. Asked by Bismarck how he thought the Germans should respond to costly French guerrilla attacks, Sheridan unhesitatingly replied: "The people must be left nothing but their eyes to weep with after the war." He advised that the insurgents be hanged, their villages burned and their lands laid waste until they begged for peace. While some Germans found this advice unacceptably cruel, Bismarck took it to heart. There should be "no laziness in killing," he ordered.

Of the three great generals who won the Civil War for the Union—the others were Ulysses S. Grant and William T. Sherman—Sheridan has remained the least known, in part because his personal papers were destroyed in a Chicago fire, depriving historians of the fine-grained research material that has brought so many soldiers of the Civil War into close focus....