Until only two years ago, northern Germany was believed to have been a no-go area for Roman troops after three legions were wiped out by German tribesmen in the Battle of the Teutoburg Forest in AD 9. The revelation that two centuries later a Roman force mounted a punitive raid deep inside the tribal areas in AD 235 has changed all that, suggesting that a soldier-emperor, Maximinus Thrax, seriously attempted to subjugate the north of Germany. The debris from the battle is scattered over a wooded hill, the Harzhorn. An archeological dig there this summer turned up 1,800 artefacts. A...