Jovian at once continued the retreat begun by Julian, and succeeded, continually harassed by the Persians, in reaching the banks of the Tigris, where a humiliating treaty was concluded with the Persian king, Shapur II.
Under Jovian, Christianity was established as the state religion, and the Labarum of Constantine again became the standard of the army.
Julian pressed forward to Ctesiphon but succumbed to a wound; and his successor Jovian soon found himself in such straits, that he could only extricate himself and his army by a disgraceful peace at the close of 363, which ceded the possessions on the Tigris and the great fortress of Nisibis, and pledged Rome to abandon Armenia and her Arsacid protg, Arsaces III., to the Persian.
The Nestorian teachers then started a great school at Nisibis (which had been under Persian rule since Jovian's humiliating treaty of 363).
As captain of the imperial bodyguard, he accompanied Julian in his Persian expedition; and on the day after that emperor's death, when the aged Sallust, prefect of the East, declined the purple, the choice of the army fell upon Jovian.