There was another ally, whose services Julian regarded himself as entitled not to request, but to command.  Arsaces, king of Armenia, though placed on his throne by Sapor, had (as we have seen) transferred his allegiance to Constantius, and voluntarily taken up the position of a Roman feudatory.  Constantius had of late suspected his fidelity; but Arsaces had not as yet, by any overt act, justified these suspicions, and Julian seems to have regarded him as an assured friend and ally.  Early in A.D. 363 he addressed a letter to the Armenian monarch, requiring him to levy a considerable force, and hold himself in readiness to execute such orders as he would receive within a short time.  The style, address, and purport of this letter were equally distasteful to Arsaces, whose pride was outraged, and whose indolence was disturbed, by the call thus suddenly made upon him.  His own desire was probably to remain neutral; he felt no interest in the standing quarrel between his two powerful neighbors; he was under obligations to both of them; and it was for his advantage that they should remain evenly balanced.  We cannot ascribe to him any earnest religious feeling; but, as one who kept up the profession of Christianity, he could not but regard with aversion the Apostate, who had given no obscure intimation of his intention to use his power to the utmost in order to sweep the Christian religion from the face of the earth.  The disinclination of their monarch to observe the designs of Julian was shared, or rather surpassed, by his people, the more educated portion of whom were strongly attached to the new faith and worship.  If the great historian of Armenia is right in stating that Julian at this time offered an open insult to the Armenian religion, we must pronounce him strangely imprudent.  The alliance of Armenia was always of the utmost importance to Rome in any attack upon the East.  Julian seems to have gone out of his way to create offence in this quarter, where his interests required that he should exercise all his powers of conciliation.

The forces which the emperor regarded as at his disposal, and with which he expected to take the field, were the following.  His own troops amounted to 83,000 or (according to another account) to 95,000 men.  They consisted chiefly of Roman legionaries, horse and foot, but included a strong body of Gothic auxiliaries.  Armenia was expected to furnish a considerable force, probably not less than 20,000 men; and the light horse of the Saracens would, it was thought, be tolerably numerous.  Altogether, an army of above a hundred thousand men was about to be launched on the devoted Persia, which was believed unlikely to offer any effectual, if even any serious, resistance.