No woman, according to New York Mayor Jimmy Walker, was ever ruined by a book. But Christopher B. Krebs, a classics professor at Harvard, makes a strong case that an early ethnological monograph, written in the first century in Latin by the Roman historian Tacitus, may have warped the cultural identity of an entire nation.

In my old Penguin translation, “Germania” — “On Germany” — runs fewer than 40 pages, but, like other comparably short documents, such as the Declaration of Independence and “The Communist Manifesto,” its influence has been earthshaking. As the Penguin translator, H. Mattingly, frankly writes in his 1947 introduction, the book is “a detailed account of a great people that had already begun to be a European problem in the first century of our era.”

“Germania” is an early work by Tacitus (circa 56-120), whose greatest achievement, the “Annals,” provides our best account of Roman history under such “bad” emperors as Tiberius and Nero. As a stylist, Tacitus is famous for his terseness, mordant wit and a prose that can be both poetically dense and grandly magnificent. Krebs, in “A Most Dangerous Book,” neatly characterizes it as “sparkling and serrated.”

“Germania” (published in 97-98) concisely describes the customs and character of dozens of loosely affiliated northern tribes but also functions as an implicit moral tract: While Romans have sunk into softness and debauchery, the tough, blond barbarians living around the Rhine are unwaveringly loyal to their leaders, fierce in battle, without interest in gold and other baubles, obedient to their gods, chaste when young and faithful to their spouses when married.

Why are these Teutons such admirable physical specimens and moral beings? In the most unwittingly pernicious sentence of his superbly readable book, Tacitus writes at the opening of Chapter 4: “For myself I accept the view that the people of Germany have never been tainted by intermarriage with other peoples, and stand out as a nation peculiar, pure and unique of its kind.”

The Germans are, in short, racially homogenous. This accounts, Tacitus adds, for their common body type: blue eyes, flaxen hair, huge frames. Moreover, since battle is viewed as the sole worthwhile activity, young warriors are intensely devoted to their band (comitatus) and will fight to the death for their leader. Drinking to excess is almost the only vice among these noble savages, though they do sometimes sacrifice human beings in their religious ceremonies.

As Krebs reminds us, Tacitus was largely unread and half-forgotten during the Middle Ages and rediscovered only by Renaissance humanists. “Germania” survived in just one manuscript. At first, Italian commentators, such as Enea Silvio Piccolomini — later Pope Pius II — viewed it as a chronicle of uncouth beastliness. These pagan tribes had no literature and no art; they dressed in bearskins and slept on the ground. But Northern scholars saw the book differently: What the ancient Teutons “lacked in cultural refinement they more than made up for by moral rectitude.” New editions and translations of “Germania” gradually appeared, and soon this “golden booklet” had established itself as the foundation work of German cultural identity.

From then on, those ancient barbarians were praised for never having been conquered by the Romans. The German language, it was maintained, possessed a deep history and continuity that mongrelized Latin tongues such as French and Italian could only envy. The ancient Aryan culture was obviously led by wise druids, inspired soothsayers and epic-singing bards. Drawing in part on their reading of Tacitus, Johann Gottfried Herder (1744-1803) and others began to promulgate a vision of a transnational Germanic people, or “Volk.” The German spirit, or Geist, took on an increasingly mystic and mythic quality. And so did Tacitus’s little book.

“From the turn of the nineteenth century,” as Krebs writes, “the Roman historian was twisted to testify to the purity not only of the mores and the language but also and increasingly of the racial constitution of the Germanic ancestors as members of the Caucasian, then Aryan, and finally Nordic race. Racially pure the Germans had been; racially pure they should be again.”

In those romantic years, the “Volkish” program embraced Teutonic folklore as chronicled by Jacob Grimm, the Northern myths as transformed by Richard Wagner into “The Ring of the Nibelung,” and various theories about racial degeneration and ethnic purity. Throughout this period, Tacitus was read as providing the template for what a true German should be, though his text sometimes needed to be slightly bowdlerized — those human sacrifices were emended as scribal errors. In 1871, the German “Volk” — long divided into separate states such as Prussia and Saxony — were finally united under the empire of Kaiser Wilhelm I.

Already a key text in German history textbooks, “Germania” took on a darker identity in the 20th century as “a Bible for National Socialists.” The “golden booklet” obsessed Heinrich Himmler, so much so that the second most powerful man in Nazi Germany ordered special forces to steal the oldest surviving manuscript from an Italian villa. (They failed.) As Reichsfuehrer of the SS, Himmler deliberately modeled his dreaded elite troops after Tacitus’s descriptions of the tightknit bands of young warriors. As he ominously proclaimed in his diary, “Thus shall we be again.” Himmler duly insisted on SS initiates physically conforming to the blond muscleman ideal, even though, as Krebs notes, Himmler himself was “dark-haired, near-sighted, and flat chested.” One Nazi wit actually dared to remark, “If I looked like Himmler, I wouldn’t even mention the word race.”

In the final chapter of “A Most Dangerous Book,” Krebs underscores that less ideologically motivated scholars, from Beatus Rhenanus, a Renaissance contemporary of Erasmus, to Eduard Norden, the leading German Lat­inist of the early 20th century, recognized that “Germania” is a text deeply grounded in its own time. Many of the characteristics Tacitus ascribes to the ancient Germans, Norden proved, were actually traditional attributes conventionally assigned to all sorts of peoples in early histories and poems.

In short, Krebs emphasizes, “Tacitus did not write a most dangerous book; his readers made it so.”

Christopher B. Krebs is a young academic, but he has produced a model of popular intellectual history. By tracing the fortunes of Tacitus’s “Germania,” Krebs shows us how scholarship both recovers and distorts the past, how ideas take on lives of their own and how our cultural beliefs bear political consequences. Along the way, he provides a powerful justification for classical studies in a time when they are often shamefully neglected. In every way, “A Most Dangerous Book” is a most brilliant achievement.

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Tacitus’s “Germania” From the Roman Empire to the Third Reich

By Christopher B. Krebs

Norton. 303 pp. $25.95