Historians have a formidable task when they try to explain why something happened. The task becomes even harder when they ask why something did not happen. Undaunted, Mark Noll and Harry Stout take this more difficult tack in their new books on the Civil War: Why did American Christians not think more deeply or act more ethically as the country faced bloody sectional conflict?

This type of history first requires demonstrating that the non-event could and should have occurred. Both authors identify moral voices crying in the wilderness—Abraham Lincoln (though he could be hard-hearted as well as humble), a few circumspect pastors and journalists, even Union Gen. George McClellan (Stout does not ascribe his infamous reticence to incompetence but to observance of the rules of limited war). Noll also brings in the never before studied perspectives of Europeans, Catholic and Protestant, who more clearly saw the flaws in American thinking about slavery, warfare, theology, and biblical interpretation. Stout cites the centuries-old Christian tradition of just-war theory.

Next, "why not" history must prove that the expected development did not, in fact, occur. Here, Noll and Stout contend with a long tradition that celebrates the revivals in North and South during the war, extols the Christian virtues of selected generals, and uncritically lauds Lincoln's prophetic vision. Widespread willingness to go easy on the North also complicates matters.

Neither Noll nor Stout would deny that emancipation constitutes a moral trump card, but this noble act—undertaken only in the middle of the conflict, as Lincoln sought to muster support for a transition to total war—does not atone for what both authors consider ...