The expansion of the Roman empire must have seemed limitless to generations of Romans. Nearing the end of the first century BC, the Romans had conquered vast swaths of territory from Gaul in the west to Anatolia in the east. Under Augustus, around 12 BC, Roman ambitions of further conquest reached new heights with important plans to conquer and expand the German frontier from the Rhine river to the Elbe river.
In short, the troublesome task of subjugating the German tribes over nearly two decades went reasonably well. But eventually Rome’s arrogance and mistreatment of the tribes they had tried to pacify, between the Rhine and Elbe Rivers, spilled over into deep resentment and eventually open rebellion.
Led by a Germanic chieftain named Arminius, who served in the auxiliary of the Roman army, the German tribes planned to take back their lands in a brilliant coordinated ambush during the autumn (likely September) in the year 9 CE. The crafty Germans realizing they were no match for the Romans in open battle, set a trap for the Romans in the Teutoberg Forest, near the area around Kalkriese Berg. This narrow wooden pass would act to stop the Romans from forming any kind of defensive formation.
Once the Romans were deep within the forest, Arminius gave the signal for Germanic warriors to attack the Roman legions who stumbled their way through the forest, truly unaware of the fate that awaited them. By the end of the ambush, the vengeful Germanic warriors had annihilated three Roman legions, in which huge numbers of men, possibly 15,000 in all perished. (Only a few hundred men escaped the carnage.)
In the wake of the massacre, the Germans proceeded to wipe out Roman garrisons and forts east of the Rhine and for years the Romans were unable to make progress in subduing the Germanic tribes. The ambush in a nutshell stopped the northern expansion of the Roman empire, convincing Augustus that the empire was probably its most practical limits of expansion.
Photo credit: The image of the Teutoburg Forest is used under the GNU Free Documentation license.
Rearview Mirror 
Excellent post and really helps us understand why the Romans went no further. It also explains why they were always having to continually guard the Germanic frontier.
At an older age, Augustus felt more the loss of lives and costs of risky expansion than he did 40 years earlier. The civil wars cost Rome an immense loss of lives. An older Augustus understood the pain of loss. Having subdued fully Germany would have a created a new frontier and interactions with other people beyond that could have created new wars. A never ending situation. Securing the frontiers was a better proposition but avenging the loss of the three legions was stuck in his mind. Germanicus was engaged in that task of revenge. And if it wasn’t for Tiberius, who also at a younger age participated in the German conquests, Rome would have had a cold Germania Province beyond the Rhine.
I appreciate your grasp of Roman History!
I can’t remember which Roman writer it is who tells it, but im always captivated by the story of Augustus wandering around his house in the nights and despondently mumbling about his lost legions and Eagles.