Why is there a Germany? Part of the answer goes back to a battle fought in A.D. 9 in the treacherous marshes and dense thickets of Teutoburg Forest, near modern Osnabrück. As described by the Roman historian Tacitus, three Roman legions led by Quinctilius Varus had crossed the Rhine from Gaul, intent on incorporating the vast area known as Germania into the empire. They were ambushed and annihilated by German tribes under the command of a warrior named Arminius. It was one of the worst military disasters the Romans ever suffered. Some years ago, archaeologists discovered the site of the battle; the debris field was a mile wide and 15 miles long. Caesar Augustus, it is said, roamed the hallways of his palace, crying, “Quinctilius Varus, give me back my legions!” Henceforth, the Romans pretty much wrote off the area east of the Rhine and north of the Alps. The line of demarcation survives to this day as a cultural divide — beer versus wine, butter versus oil, Germanic versus Romance.

So the victory of Arminius established certain facts on the ground. In his fascinating new study, “A Most Dangerous Book,” Christopher B. Krebs draws attention to another part of the explanation for Germany’s existence — the role played by Tacitus himself, through the influence of a small volume called “Germania.” Think of it as an ancient exercise in social anthropology. The Romans had been bedeviled for years by the motley tribes they lumped together as Germans. Tacitus set out to describe them.

In his telling, the Germans possessed “fierce blue eyes, tawny hair, huge bodies.” They prized freedom, scorned luxury and esteemed military courage above all else. They were a people of sturdy values for whom “good laws” were no substitute for “good habits.” In the land of the Germans, Tacitus writes, “nobody laughs off vice; and to corrupt and to be corrupted is not called ‘modern times.’ ” Pointedly, he observed that the Germans were “not tainted by intermarriage with any other nations” but rather existed “as a distinct unadulterated people that resembles only itself.”