When you consider the loses the Romans just shrugged off during the 1st and 2nd Punic Wars, losing 2 or 3 legions does kind of pale in comparison.
The Roman force was on the move with all its baggage and households (and camp followers), which is weird behavior. Perhaps the objective was to winter over in a less remote or more defensible location.
The adversary Arminius was from the Roman auxiliaries and had served Rome, but turned coat. He understood his enemy very well, and of course it was a surprise attack.
The punitive expedition to hunt him down along with his followers ran into an ambush, but discipline and formation held; Arminius thought he was on the verge of a second great victory (and massacre), and shouted taunts at the Roman commander. He wound up having to flee for his life, and his forces were either slaughtered or scattered.
A few years later, in hiding deep in barbarian territory, he was murdered by members of his own family, probably at the behest of Rome, and for cash.
Meanwhile, the Romans reoccupied the lost territory — it was hardly “The Battle That Stopped Rome” as the recent-years book claims. The modern understanding of the full extent of Roman occupation has continually been expanded to the east and north, during the postwar period, likely as a result of the changing needs of modern politics.