So after Trajan went through all this trouble to conquer Mesopotamia, his successor Hadrian immediately abonded the province. I know Hadrian was all about the defensibility of the empire, but was the region really this easy to conquer? Was maybe trying to sell it back to the Parthians or setting up a client kingdom not an option?
I think not. This withdrawal wasn't even a great pay off since Parthians and Sassanids launched several attacks across the Euphrates in the following years. IMHO, establishing fortresses in the Western Zagros passes would have been a more coherent option for defense than the slow, easily ctossable Euphrates.
I suspect the real reason for Hadrian litteraly giving back Trajan' conquests lay elsewhere.