For Persia to be defeated three times by the Hephthalites might seem as bad as it gets: but over in the west, in the Roman empire, things were far worse. The rot began in AD 378, when the Visigoths utterly defeated the Roman emperor Valens at Adrianople (Edirne in modern Turkey). The Romans were forced to give them land to settle on, pretending to themselves that they were in some way "allies".
Both Persian and Roman empires were experiencing military defeat - followed by demands for territory and "protection money" from the winners. But the Romans were not only paying out huge sums (nine tons of gold in one instance), but had been forced to invite the invaders to settle inside their territory, enlist them in the Roman army, and give them positions of power and responsibility. The Persians too had been forced to pay money, and had also lost territory.
Roman disasters: end of the western empire
The Roman empire had finally been officially split on the death of Theodosius I - his immature and inept sons were given half each: Honorius the west and Arcadius the east. Both faced similar threats - but the east survived and the west didn't. Constantinople survived the Ostrogoths in 399, while Rome was sacked by the Visigoths in 410. In the 440s, the Huns under Attila failed to take Constantinople, but plundered the western empire with impunity until Attila's defeat and drunken death in 451.
Attila, the scourge of God (FLAGELLUM DEI)
The eastern emperors paid the Huns to go away. At about the same time, they were also paying the Persians, while encouraging the Hephthalites to keep them busy. By the 470s Rome was effectively ruled by the Goths - it was a mere formality when in 476 Odoacer forced out the last Roman emperor in the west, the ridiculously named Romulus Augustulus. The east continued, and Constantinople was not finally taken until AD 1453 - why?
Part of the triple land wall of Constantinople built by Theodosius II in 413-414.
- Most of its territory was still intact (much of the west had already been hived off by various invaders - Vandals in Spain, Burgundians in central France, Visigoths in western France, Huns in Germany: Britain had been abandoned to Angles and Saxons.)
- Constantinople was protected by massive new walls, and a resident army (which was not under the control of Goths, as the western forces had been).
- Bureaucracy: an experienced civil service ensured continuity, whoever happened to be emperor (and however corrupt he might be).
- Wealth. The revenue from taxation of wealthy provinces - Asia Minor (Anatolia), Syria and Egypt - ensured that there were sufficient resources to bribe barbarians to stay away, and to pay the 100,000 strong army (however extravagant the emperor may have been).
- The Persians, who could have taken advantage, were too busy fighting the Hephthalites.
Persia: slow recovery begins under Balash (Valakhsh) (484 - 488)
After Peroz's final defeat to the Huns, the Sasanian empire was in a complete mess, politically, militarily and economically. It took many years to get back to "normal". Two of the ancient Parthian families stepped in - the Karen and the Suren. They started by making Balash, Peroz's brother, king. He wanted to start by driving the Huns out of Sasanian territory. But he couldn't afford another war, so he had to buy peace, at too high a price, the nobles believed, though all the Persian prisoners were freed. He had to agree to paying "protection money" every year.
He also made peace with Armenia, where there'd been conflict since Yazdegird II's brutal suppression of Christianity. Balash let them be Christians if they wanted to, thus making sure that Armenia could again be a secure ally against invasion via the Caucasus; very necessary as Turkic invaders were already on the move into Armenia and threatening Mesopotamia.
It was during the reign of Peroz or Balash that the Church of the East (aka Nestorian or Assyrian church) became the only acceptable form of Christianity in the Sasanian empire.