It was his intention to attack Ctesiphon with two separate armies, acting upon two converging lines.

While he himself advanced with his main force by way of the Euphrates valley and the Nahr-Malcha, he had arranged that his two generals, Procopius and Sebastian, should unite their troops with those of the Armenian king, and, after ravaging a fertile district of Media, make their way towards the great city, through Assyria and Adiabene, along the left bank of the Tigris.

It was a bitter disappointment to him when, on nearing Ctesiphon, he could see no signs and hear no tidings of the northern army, from which he had looked for effectual aid at this crisis of the campaign.

We have now to consider how this failure came about, what circumstances induced that hesitation and delay on the part of Sebastian and Procopius which had at any rate a large share in frustrating Julian's plans and causing the ill-success of his expedition.
It appears that the Roman generals, in pursuance of the orders given them, marched across Northern Mesopotamia to the Armenian borders, and were there joined by an Armenian contingent which Arsaces sent to their assistance.

The allies marched together into Media, and carried fire and sword through the fruitful district known as Chiliacomus, or "the district of the Thousand Villages." They might easily have advanced further; but the Armenians suddenly and without warning drew off and fell back towards their own country.