Julian's invasion of Mesopotamia in 363 AD was the last great expedition mounted by a Roman army beyond its own borders. It was also a disaster, resulting in Julian's death, the surrender of the army and the loss of Roman territory. Much has been written on this subject, but it seems to me that many historians have failed to analyse the expedition in great depth. Too many seem to get caught up in trying to understand what flaw of Julian's character led him to invade. But to understand what actually went wrong, the question is not why Julian invaded Persia. Whether he did it for glory, to win the loyalty of the Eastern army, to avenge the loss of Amida, or for one of a number of other reasons matters not. What we must ask ourselves is what was Julian's objective? To understand the failure of the expedition this is, in my opinion, the critical question. It is a difficult one to answer.
Our main source, Ammianus Marcellinus, provides us with a detailed account of the invasion (one he participated in), but it is an account lacking in certain areas. Ammianus was not a member of the high command. Also Ammianus wrote years later when the outcome of the expedition, and its consequences, were well known. An air of defeatism seems to hang over the narrative. Ammianus spends a great deal of time listing various omens and portents which signalled Julian's doom rather than discussing the mission's objectives.
Unfortunately there are lacuna in the text at critical moments. Perhaps the most important being at the end of Ammianus 24.7.2 where, it would seem, we lose an account of Procopius' failure to link up with Julian, Sapor's attempts to negotiate and some discussion on whether to burn the ships or not. This all occurs as Julian and his army are encamped before Ctesiphon, perhaps the most decisive moment of the entire expedition.
Due to the failings of Ammianus' work we are left with several questions which are either extremely difficult or impossible to answer. Why exactly were Procopius and Sebastianus sent with 30,000 men into Armenia? Why did Julian march down the Euphrates as oppose the Tigris? Why, having reached Ctesiphon, did he not try to take the city? Why were the ships burned? Where was Sapor throughout the invasion? And perhaps most important of all, what did Julian hope to achieve?
Ammianus' history does not allow us to answer these questions satisfactorily. Some historians have attempted to make guesses, but these, no matter how well argued, cannot be confirmed. The last question in particular is critical. Did Julian hope to conquer Mesopotamia? Did he hope to bring Sapor to battle and defeat him, as he had with the Alamanni at Strasbourg? Did he hope to establish a friendly regime under a ruler of his choosing? Or did he merely hope to catch the Persians off-guard and sack Ctesiphon, winning the loyalty of the Eastern legions by allowing them to plunder that great city?
Despite having given this subject great thought, I do not claim to know the answers to these questions. I suspect that Julian turned from Ctesiphon because he hope to find and defeat the Persian army in battle. Whether he would have been successful in this is uncertain, but, as it was, his death intervened. This is just a guess, however, and I remain to be convinced. Unfortunately going by the information we have guessing would appear to be all we can do. But this should not discourage us from trying to analyse the invasion in depth, nor should we resort to lazy conclusions, merely writing the episode off as a disaster or a mistake. Without knowing Julian's objectives we cannot say if the invasion was a mistake because we cannot know what he was attempting. The final outcome might be called a disaster, but much of this occurred after Julian's death. I plan to examine the events of the retreat in more detail at a latter date and so shall say no more on that subject here.
Further reading: Austin, N. J. E., Julian at Ctesiphon: A Fresh Look at Ammianus Account, in Athenaeum 50, 1972. Kaegi, W. E., Constantine's and Julian's Strategies of Strategic Surprise Against the Persians, Athenaeum 59, 1981. See also Ammianus Marcellinus' Res Gestae.