In 358 Shapur II was ready for his second series of wars against Rome, which met with much more success. In 359, Shapur II invaded southern Armenia, but was held up by the valiant Roman defence of the fortress of Amida, which finally surrendered in 359 after a seventy-three-day siege in which the Persian army suffered great losses. 

In 363 the Emperor Julian, at the head of a strong army, advanced to Shapur's capital city of Ctesiphon and defeated a presumably larger Sassanian force at the Battle of Ctesiphon; however, he was unable to take the fortified city, or engage with the main Persian army under Shapur II that was approaching. Julian was killed by the enemy in a skirmish during his retreat back to Roman territory. His successor Jovian made an ignominious peace in which the districts beyond the Tigris which had been acquired in 298 were given to the Persians along with Nisibis and Singara, and the Romans promised to interfere no more in Armenia.

According to the peace treaty between Shapur and Jovian, Georgia and Armenia were to be ceded to Sasanian control, and the Romans forbidden from further involvement in the affairs of Armenia. Under this agreement Shapur assumed control over Armenia and took its King Arsaces II (Arshak II), the faithful ally of the Romans, as prisoner, and held him in the Castle of Oblivion (Fortress of Andməš in Armenian or Castle of Anyuš in Ḵuzestān).