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Rome had expansionist policy from its early days since the Dacian conquest by Trajan. During these several centuries, Rome took on powerful and established powers such as Carthage, Macedon, Selucid kingdom and so on. It didn't have much difficulty in conquering loose barbarian tribes either, such as the conquests of Gaul and Britain. However, Rome never managed to conquer the Germanic tribes, and the Rhine Danube frontier was always a potential trouble spot for Romans. Why is that?
Now, Romans certainly tried advances in Germany¹, but they were unsuccessful. Most notable example of this would be the disastrous Battle of the Teotoburg Forest. But I am interested in the reasons at a higher level.
Also, it is curious that they never forayed significantly in this region. Mesopotamia was a key area for Romans, and they had at least managed to conquer it once; even though they couldn't even hold it for a decade (Trajan's Mesopotamia). There is no analog for Germany. Was it because Romans were never interested in Germany?
If the scope of this is too broad, then I am interested in the expansionist phase of Roman history, possibly from start of second century B.C. to the end of Trajan's reign.
¹ By Germany I mean what Romans meant by Germania.