I noticed that the Roman Empire never went that far into Germany. Why is that? Also (and this may be a weird or loaded question) does this somehow give justification to the Nazi claim of racial superiority?**I'm not saying they were superior, I'm just wondering if the Nazis used this to make their claims.
How about Germany was pretty much impenetrable forest in Roman times inhabited by implacably hostile Germanic tribes. Look up the Battle of the Teutoberg Forest to understand why they did not penetrate far. The Romans lost three entire legions wiped out almost to a man in that battle. There are echoes of that forest remaining. I always tell my wife that we are in goblin territory when we drive through some of the thick pine forests near our house. I can just imagine what foot-bound infantry thought when they were getting ambushed out of the seemingly endless forests north of the Rhine.
I think it must have been difficult to keep such a long imperial border constant with the barbarians to the north. The map attached shows Rome in 117 A.D. One thing that stands out is that at least part of the border in Germany was along rivers, such as the Rhine and the Danube. No doubt these provided at least some level of natural protection from the barbarians. Also, the Romans may simply have reasoned that there weren't any great cities to conquer or resources to obtain there that they couldn't get elsewhere (notice they also didn't go too far south into Africa, either).
Dio describes it as the edges of the earth. So I looked up what that meant and found this.
Edges of the earth: the parts of the world where, according to the myths of the ancient Greeks and Romans, fabulous creatures and savage barbarians lived. These legendary beliefs influenced geography and can still obstruct our understanding of ancient ethnography.
I think it must have been difficult to keep such a long imperial border constant with the barbarians to the north.
The Roman lines on their northern border are the classic example of an elastic defense-in-depth. They made extensive use of river lines because they are easily defensible, especially because there were very few bridges and none on the Rhine. Caesar built the first bridge over the Rhine and it was only a temporary construction. The Sahara provided a buffer in Africa and the Parthians stopped Roman advances to the east. The argument could be made that the map shows the natural limits of Roman Hegemony and they could not have gone farther even if they had put serious effort into it. The administrative challenges of running the empire they had were enormous, many modern countries could not do it but the Romans managed their empire for almost a thousand years without modern communications. That is fascinating all by itself.