Most disastrous Roman Military Defeats

[Robert Vermaat]

Rome's boundaries were decided by Geography, not military defeats !! 8)

I'm not sure I agree wholly there.
I mean, every empire is limited by the ability to raise forces that expand and defend the borders. That ability is limited by political and economical circumstances, which vary over the period of time in which an emoire exists. Sometimes an empire simply has bad luck, when a general gets ill just before a major batttle, or when a meteor falls on the capital or something like that. 8) More usually though, economics define demography (manpower), social issues influence politics (civil wars) and culture (advanced medical science vs. plagues etc.) to name just a few.
But in fact, when an empire is close or across the limits of its abilities to expand, there will be defeats. So in that way, defeats really decide boundaries, even when defeats are (mostly) due to several factors.