Excellent reasons all...but it comes to this. The Germans tribes were too tough and the expense in blood and money was just not worth the effort to the empire. I seriously doubt Rome could have taken the tribes at any expense anyway in consideration of the climate and the terrain. It would have been a campaign that would have drained the Empire empty. Besides, all they had to do is wait and eventually the tribes would come to Rome...as they, in essence, ultimately did in 405 AD..hehe

Was it just terrain that enabled tribes to defeat the army? At the time, how would you deal with the threat? Just raze their territory from one end to the other to avoid fighting them on their terms, or was that too not practical ;)

It is easier to conquer a place when there is a central authority as the people accustomed to obey a central figure as long as their life style is not changed. For that reason it was easier to conquer the mighty Persian Empire than to conquer Germany. Against Persia you only need to destroy the armies of the Empire, the land would surrender to the victor by itself. But against Germany you need to defeat every single tribe again and again.

Of course the same should have applied to the various Gaullic tribes also when Caesar conquered them. Although it is true what you say about a centralized state, every variety of political constitution must have its own individual weaknesses. While it is true the Germanic tribes were at that time able to terrorize the Gauls because the Gauls had become less warlike, Rome could have used the same divide and conquer strategy on the Germans that Caesar used in Gaul. By the way, there were a few German tribes in the south that were allied to Rome as it was. About the Germans around the time of Hadrian, have you read "Agricola and Germania" by Tacitus? The Germania part describes the Germans in fair detail. Agricola was a Roman general who pacified Britain. About the Gauls visa-vis the Germans, I have read that the Boi were a Celtic tribe which lived among the Germans and kept them in fear. Bohemia is named after them.

Germans also had one advantage over Gauls. Gauls in France did not have the great Germany forest. Which makes hit and run strategy more easy. I did not read "Agricola and Germania" yet. I like to in the future.

I think another reason that Rome did not conquer Germania is that then armies of Rome were not led by a commander of genius like Julius Gaius Caesar.

I agree that a good general like julius Ceasar would have helped, but if you lose a big chunk of your army in one go i n fact your entire army in a particular sector you have to pull forces from else where until you can replace the losses and then you are on the back foot and there is never enough time money or manpower,so you have to draw a line and stand behind it.[try playing rome total war you will soon know what im talking about].

While reading over a book on the Roman world from the 2nd century BC to the third century AD, I came across a characterization of Augustus' policy on the size of the Roman army. There wasn't any more than a sentence or two on this and I didn't write it down. The size of the army was reduced to less than half during Augustus' rule (From 60 to 20 something legions). Other changes which seemed to account for this were ones that increased the expense of keeping soldiers in the army. One was a retirement pension which was begun during Augustus' reign. Could it be that the failure to replace those 3 legions wiped in Germany was due to there being pre-existing plans to reduce the size of the army? Again, I have to wonder if the reasons for the empire not conquering Germany were domestic and political.