Give Me Back My Legions by Harry Turtledove

Around the year AD 6 the Emperor Augustus decided that the troublesome German tribes north of the Rhine should be pacified and brought within the Empire. The obvious choice to lead such an expedition was his son-in-law, and designated successor, Tiberius. However, he was currently commanding the army in stamping out the last embers of revolt in Pannonia. Augustus, therefore, gave the command to Publius Quinctilius Varus husband of his great-neice Claudia and in doing so made the most disastrous mistake of his reign. Varus was an experienced administrator, he had recently held the governorship of Syria one of the richest provinces of the Empire, but had minimal military experience. Why Augustus made this decision is a mystery. He could have waited until Tiberius was finished in Pannonia or transferred him to the new project leaving his subordinate generals to continue mopping up the rebels. There were, of course, several other experienced military men who could have been given the command but instead Augustus chose a career bureaucrat and the Legions and the Empire paid a terrible price for his error. Varus was put in charge of three legions, twenty thousand men with irreplaceable front line battle experience, the bedrock of the Empire, and threw them away through a combination of ignorance and arrogance. At this time the Roman Legion was the most deadly military machine in the world but only if they were able to deploy in their well-rehearsed formations. It was the function of their Generals to ensure that any battle took place in terrain that allowed for this. Varus, however, was simply not capable of comprehending this and ordered his men along a route that trapped them on a narrow track hemmed in by forest that made deployment impossible. The result was a massacre and meant that Germany never became part of the Empire. Harry Turtledove has written an excellent book combining historical characters and real incidents with a well imagined narrative.