More than 140 years ago, Mark Twain observed that the Civil War had "uprooted institutions that were centuries old, changed the politics of a people, transformed the social life of half the country, and wrought so profoundly upon the entire national character that the influence cannot be measured short of two or three generations." In fact, five generations have passed, and Americans are still trying to measure the influence of the immense fratricidal conflict that nearly tore the nation apart.

In The War that Forged a Nation, Pulitzer Prize-winning historian James M. McPherson...