[5] Arshak participated in the Roman emperor Julian's ill-fated campaign against Persia; after the consequent Perso-Roman Treaty of 363, Armenia was left to fend for itself against a renewed attack by the Persian king Shapur II.
[6] Faced with defections and rebellions among the Armenian nobility, Arshak was lured to Persia for peace negotiations with Shapur, after which he was imprisoned in the Castle of Oblivion in Khuzistan and is said to have committed suicide in captivity.
[3][6] Arshak's reign was followed by the conquest and devastation of Armenia by the Persians, although his son and heir Pap managed to escape and later ascended to the Armenian throne with Roman assistance.
Around 338, the Sassanid king Shapur II launched a war on Rome and her allies, joined by a wave of persecutions against Christians living in Persia and Mesopotamia.
[12] According to another view, Arshak was enthroned as early as 338/339 (possibly at the request of Constantius II) following Shapur's defeat against the Romans near Nisibis.
[15] He may have adopted a neutral position early in his reign, followed by a possible reconciliation with Persia, and then an abrupt pivot towards Rome in 358 because of the emperor Constantius II's concessions.
[16] Arshak II, like his father, pursued a policy strongly in favor of Arianism, which led to a falling out with Catholicos Nerses.
[18] Arshak's relations with the Armenian nobility also soured, leading him to order the assassinations of prominent nakharars, the extermination of several noble houses (such as the Kamsarakans) and the confiscation of their lands.
[18][b] He attempted to shore up his rule by founding the city of Arshakavan in Kogovit, which, according to Faustus[22][23] and Movses Khorenatsi, he populated by granting amnesty to any criminals that would settle there, as well as debtors, slaves, and others.
[24][18] Arshak was summoned by Constantius II to Caeserea in 360[26] and warned to remain loyal to Rome, after which, per Ammianus Marcellinus, the Armenian king "never dared to violate any of his promises.
[3][30] Faced with this desperate situation, Arshak agreed to go to Persia for peace negotiations with Shapur c. 367 after receiving guarantees for his safety.
[32] An almost identical account of Arshak's death is given in The Persian War of the Byzantine historian Procopius, who cites a certain Armenian history and may have been (but, per Garsoïan, was not necessarily) familiar with the work of Faustus.
[35] M. L. Chaumont characterizes Arshak as "weak and indecisive,"[3] while Vahan M. Kurkjian writes that although the Armenian king did not lack "intelligence, courage and will power [...] he committed many mistakes and cruelties which overshadowed his virtues and contributed to his tragic end.
[41] Historian Albert Stepanyan argues that Pap was in fact Arshak's son, but that he was initially legally regarded as Gnel's son, as Arshak had married Paradzem in an Iranian-style levirate marriage called stūr ī būtak or čakarīh, whereby a childless widow would marry one of her late husband's agnatic relatives to provide her deceased husband with an heir.
[42] According to Stepanyan, it was only after the death of Olympias that Parandzem was made a full royal consort and her son Pap was recognized as crown prince of Armenia.
[44] Arshak apparently had another son, not mentioned by name in the histories of Faustus and Movses Khorenatsi, who may have fathered Varazdat, Pap's successor as king.