The reign of Khosrow I of Persia, and rivalry with the Byzantine Empire.
Khosrau Anushirawan ushered in a golden age of Iran, but only after his father Kavadh suffered through the near collapse of the empire. Once he broke free from a controlling minister and radical religious reformer, Kavadh realized that the empire needed to change.
Kavadh asked his allies in Eastern Rome for help getting Iran back on its feet. The Romans' replies were not only unhelpful - they were insulting. By the time Khosrau inherited the throne, resentment and war had turned the delicate alliance with Rome into an open rivalry.
Once the chaos settled, Khosrau enjoyed an unprecedented era of peace. He brought reform to the army and the economy, invested in a great center of learning, imported knowledge from around the world, and earned his new title of "The Immortal Soul."
Iran and Rome had agreed to an Eternal Peace, but tensions between them proved too great and Khosrau decided to invade while Justinian's guard was down. His army swept into Rome practically unopposed, and he made a mockery of Justinian at every opportunity while treating himself to a grand old time pillaging and parading across the Roman border.
Plague had brought an end to Khosrau's war against Justinian, but Justinian's nephew soon reignited the rivalry. Khosrau was at the peak of his political power and eager to crush this young upstart personally... but old age had also crept up on him.