The 20th century is often described as the most bloody in human history, but rarely are the red drops as carefully catalogued as in Jonathan Glover's "Humanity: A Moral History of the Twentieth Century." Less a history book than a study in applied ethics, Glover attempts to analyze what remains of the Enlightenment after the gore of the past 100 years. The French Revolution, Rwanda, Nazi Germany, Soviet Russia, Hiroshima, World War I--Glover cringes from each of these in turn.

"Humanity" is ultimately little more than a compendium of conventional wisdom. Glover offers little clear explanation of why these atrocities happened, content with describing them as the outcome of a disastrous meeting between the darker aspects of "psychology" and new destructive technologies. Even his moral lessons are confusing yet predictable; he observes that having a strong sense of identity can help people resist committing terrible acts, yet that at times a strong identity can actually be forged in the commission of horrors. Religion plays a minor role in his analysis; problems of character are considered in terms of identity rather than faith. More attention to the history of these terrible regimes--rather than simply repeating how terrible they were--would have deepened not only the historical but the moral weight of Glover's book.