Adrian Goldsworthy has produced an interesting volume on the 700 year rivalry between Ancient Rome and Ancient Parthia and their replacement, The Sasanian Persians. The titles is a tad misleading, because although the Parthians ruled over what was Persia, nobody ever called them Persians, although the Sassanids were. Goldsworthy is upfront about not being super detailed, because of the large time span, and further is at pains to explain what was going on in the Parthian and Sassanid empires because they are so poorly documented. Despite that, this is both a good introduction to the subject and a useful reference to the ebb and flow of relations between these empires. Goldsworthy makes the point that no Roman leader or emperor actually intended to conquer the Parthians and Sassanids, and likewise, until 7th century AD, neither did they. The rivalry seems to have taken the form of never ending dueling over the border areas between them. The final war between Rome the Sassanids likely weakened both to where neither were able to resist the Muslim conquests, although Rome fared a bit better, the Sassanids ending up conquered by the Arabs.
Goldsworthy also tries as much as possible to consider all sides' motivations as he writes in his introduction: "Stepping back and studying what happened over such a long period immediately encourages caution about assuming that attitudes were simple and unchanging. A major weakness of these debates is that they have tended to speak of Roman imperialism and Roman frontiers while ignoring the part played by other states."

Some take aways:
-Armenia was more important to the conflicts between Rome and the Parthians/Sassanids than is usually thought of when talking about Rome and it's adversaries. Much like Otto Bismark's bon mot about "Some damn thing in the Balkans", some damn thing in Armenia seems to spark off conflict between Rome and the Parthians/Sassanids again and again.
-The Sassanids built extensive border fortifications to guard their northern flank, that is just now being more thoroughly investigated.
-The 6th and 7th century wars between the Eastern Romans and the Sassanids forced a convergence of military practices that seems to have ended up with the Romans resembling the Persians more than anything else.
-The Muslim conquests were not a sure thing, but were a matter of being in the right place at the right time and taking full advantage of it.

All in all, another good volume from Goldsworthy. ( )

ambition of this author to render comprehensible the relationship between Rome and the other super-power of the age Persia should be praised, and he has been largely successful. It is a highly complex history, not least because of the various political leaders ruling over a vast territory. Persia as a region at this time was ruled by the descendants of a general of Alexander the Great, as had happened in Egypt, so this is not country of Xerces and Darius. However, that said I now feel I have a better understanding of how Rome conducted its diplomacy with equals, something we rarely see with most histories of her expansion.

I would recommend this to anyone with an interest in the history of Rome, although I would probably advice to obtain a physical book as you will need to reference maps etc fairly frequently. ( )