The only serious external wars under Anastasius were on the eastern front. Acute problems emerged with Persia for the first time since the 420s. Religious issues played only a marginal role in the conflict. There was a large Christian population in the Persian Empire, which had grown after the triumphs of the Monophysites at the councils of Ephesus in 431 and 449, when many Nestorians fled from the Roman Empire. In the course of the fifth and sixth centuries the Nestorians created numerous churches, the largest religious network in Persia outside the framework of the official Mazdaeic cults.64 However, despite some signs of intolerance shown by the Persian priestly caste, outbreaks of persecution and martyrdom in Persia during this period were due to the conflicts of Nestorians with Monophysites, not of Christians with the Sassanid authorities.65 In the later sixth century the treaty between Rome and Persia of 562 guaranteed tolerance to the Christians in Persia, including the right to build churches, conduct services, and sing hymns of praise to God.66 This tolerance continued into the seventh century under Khusro II, and Nestorians took advantage of the political order provided by the Sassanian Empire to proselytize in distant Asia, reaching China as early as 635.67
Peace between Rome and Persia was maintained though most of the fifth century on the basis of diplomatic understandings. Procopius records a story that the emperor Arcadius, on his deathbed in 408, had entrusted his infant son Theodosius II to the guardianship of the Sassanian king Yazdgird I (399–420), as a device to protect against usurpation.68 Socrates noted that amicable relations between Roman and Persians were sustained by Yazdgird's tolerant, even sympathetic, attitude towards Christians, although this was undermined by the hostility of the magi, and attitudes hardened under Yazdgird's successor Bahram V Gur (420–39) (Socrates 7.8.1–20; Theodoret 5.39). A war broke out in 421 but ended a year later with a fifty- (or one hundred-)year truce in favor of the Romans.69 These terms were renewed around 441 and an undertaking was added by both sides not to build additional fortifications close to the border that separated the Roman from the Sassanian Empire (Procopius, Bell. Pers. 1.2). In political terms the main factor in Roman–Persian relations was their common fear of the Huns. The Persians paid regular tribute to the Kidarite and Hephthalite Huns, who occupied a strong position at Gorgo (Gurgan), on the plains of Hyrcania east of the Caspian and adjacent to the Sassanian frontier. To the west of the Caspian Sea the Caucasus Mountains were a defensible barrier running to the Black Sea, important both to the Persians and the eastern Roman Empire. In the sixth century John Lydus claimed that Jovian had almost reached an agreement with Sapor II, after the defeat of Julian, that both sides would contribute to the costs of securing the Caucasus, and that the Persians built and sent garrison troops to a fortress Biraparach, to prevent barbarian incursions (John Lydus, de mag. 3.52). In 467 and again in 484, at the death of the king Peroz (459–84), Rome aggravated relations by refusing certain payments to Persia.70 However, the main source of grievance was the status of the vital frontier city of Nisibis, where import and export dues were imposed on trade between the two empires:
Jovian preferred peace above all else and for this reason ceded control of Nisibis to the Persians for one hundred and twenty years. It was to be restored to its masters at the end of this period, which came at the time of the Roman emperor Zeno, but the Persians did not want to return the city and this gave rise to the quarrel. (Ps-Joshua, Chron. 7, trans. Watt)
During the 490s Anastasius argued that Nisibis should be restored to the Romans to provide revenue to support their wars in Ethiopia and against the barbarians in Europe.71 Meanwhile the Sassanian king Kavad 1 (488–97/499–531), after overcoming serious internal rebellions early in his reign, renewed demands for a payment to be made. When these were refused, war broke out.72 The Persians took control of Armenia and northern Mesopotamia. Anastasius responded by sending large forces to Edessa, where the Goths and other foederati proved a particularly unwelcome burden to the local population (Ps-Joshua, Chron. 86, 93–96; see pp. 184–5).
In 504 the Romans recovered Amida, the main city of northern Mesopotamia, by a payment of 1,000 pounds in gold, and two years later a seven-year truce was agreed. Anastasius now began to fortify the village of Dara, a well-watered position on a tributary of the river Chabur, as a counterweight to the frontier city of Nisibis.73 The former village was equipped with baths, cisterns, churches, an administrative palace, and a column in honor of the emperor, as well as covered porticos and storehouses, and was renamed Anastasioupolis. Dara remained a standing provocation to the Sassanians, as it conspicuously breached the terms of the agreement of 441, but the seven-year truce stretched on beyond the death of Anastasius and into the reign of Justin (see pp. 371–2).
Neither side was strong enough to defeat the other, or sufficiently well-equipped and well organized to mount sustained campaigns in its rival's territory.74 The centers of power on both sides lay far distant from their mutual frontier. Both empires were also seriously challenged by other powers: the Sassanians by the Hephthalite Huns, and in the later sixth century by the western Turkic tribes who were impinging on their eastern frontier; the Romans by continued threats and challenges from barbarians in Europe and from the Vandal settlement in Africa. Thus both had a vested interest in maintaining the peace, which had lasted through most of the fifth century. The hostilities under Anastasius interrupted this pattern only to a limited degree. Higher levels of belligerence later in the sixth century reflected the personalities of the respective rulers, Khusro I and Justinian.