I’ve recently gotten through this period in the “History of Byzantium” podcast series. Justinian had just recently retaken many of the old western Roman provinces, not least of which was Rome itself, and the breadbasket of North Africa. Things were finally looking good for the Romans again when the plague hit. The armies were unfortunately overextended and now half their ranks were dead or dying. It was an irresistible target for the Persians, the steppe horsemen and the Goths which ultimately weakened them all just in time for the Arab invasions.