Romans win at Teutoburg

Well that was the classic problem wasn't it, that once the huge pool of slave labour dried up the economy started declining. How far does one continue to conquer? After the Romans hit the Vistula there lay endless plains and marshes to the East and the inhospitable frozen lands to the north. The buck had to stop somewhere. I think any notion of the Romans exceeding the limits of the Vistula would be ridiculous.

LOL, in the later Empire no one had to go out of their way to encourage that. If the Emperor took his eye off the ball for just a little while he had more than he could deal with.

Well that's one thing that impresses me about Byzantium. Inspite of the loss of a massive pool of slave labour which had traditionally kept the Roman Empire so economically strong, the Byzantines had to find another way of tapping their own resources. The theme system, creating a hardy and productive class of smallholders in Anatolia, accomplished this brilliantly. The late Roman prefecturates were simply not economically and militarily efficient enough to achieve such a utilization of manpower.

I don't know what you mean by well-trained. Those Germans were not professional soldiers. As mercenaries, they were recruited as they were, and not given training by the Romans.

This article gives good background on the German/Roman conflicts:

http://www.livius.org/te-tg/teutoburg/teutoburg01.htm