[pierrem15]

Not sure I agree entirely-- the fact that the Parthians were not as great a threat as the Sassanids is undoubtedly true, but the fact that the threat was real enough to send Lucius Verus with a substantial army to the East (draining resources from the Danube frontier) shows that the fundamental strategic threat to the Empire was already serious at the time Marcus Aurelius.

The fact that both campaigns ended indecisively also did not bode well for the Romans. Only a little more than sixty years before Trajan was able to mop the floor with the Persians, and (with difficulty) to acquire Dacia.

In retrospect it appears Hadrian's decision to call off Trajan's Persian campaign and build enormous and costly frontier fortifications may have been a grave mistake.

[SunkenCiv]

Hadrian did what he did because he was only interested in boy’s anuses. Regardless, considering what turmoil the people of the Roman Empire were put through, it was obviously cohesive and durable; from the death of Hadrian to the fall of Rome was nearly three centuries. And the eastern capital didn’t fall for another millennium.

Romans and Barbarians: Four Views from the Empire’s Edge, 1st Century AD
Derek Williams
http://www.amazon.com/exec/obidos/ASIN/0312199589/sunkencivilizati

Frontiers of the Roman Empire
Hugh Elton
http://www.amazon.com/exec/obidos/ASIN//0415692555/sunkencivilizati