Khosrow II had two reigns. His first was interrupted by Bahram Chobin, who was a great Sassanid General who rebelled after being thrown out of power by Hormizd IV.
This was the first time that the Sassanids were being challenged by the house that had ruled the Parthian Empire, the previous masters of Iran. Bahram Chobin while highly skilled and attempting to use the Zoroastrian religion and its tales of conflict and change arriving at the end of Zoroaster’s Millenium. In spite of his efforts, Baharm Chobin was defeated as Khosrow II retained the allegiance of important clergymen and nobles. He also had help from the Byzantine Emperor, Maurice.
The Eastern Romans went out of their way to help Khosrow II as it meant that they would regain important territories on the Eastern border. When Bahram Chobin was beaten, genuinely helpful and valuable relations began to emerge between the Sassanids and the Byzantines.
However, when Maurice was killed in a coup instigated by the army and Emperor Phocas came to power, Khosrow II began invading Byzantine territory, under the pretense of avenging his ally. Was this honest or was he just justifying the chaos to attack and gain Byzantine territories?
Emperor Maurice employed realpolitik on the Sassanids and Khosrow II did the same here. However, Emperor Maurice stayed true to his word and did not annex more than what was given to him by the Shah.
However, Khosrow II did not march straight to Constantinople to change the ruler of the Byzantines. He began taking territories while the Byzantines were fighting amongst themselves. It would take until Heraclius and his father sailing from Carthage to oust Phocas and begin the long process of attempting to expel the Iranians.
Khosrow II was extending the empire much further than the Parthians had done and in 620 AD, it was at its biggest extent since the Persian Empire that faced Alexander the Great. Syria, Egypt, parts of Yemen, areas outside traditional Persian control were not under the adminstration of the Shah.
However, within eight years, Khosrow II would be killed by his son and all the gains would be given up. The Muslims would sweep Iran and the Islamic era of Iran would begin.
Ultimately, Khosrow II could have helped Heraclius. However, he decided to continue on with capturing Byzantine territory. He didn’t really have a reason to stop, considering that he had used the discontent of the Jews to revolt against Heraclius and gain control of Jerusalem. He also managed to retake Egypt and add it to his empire.
Khosrow II was probably an ally of Maurice, not the empire. He thanked him for fighting against Bahram Chobin and had good relations with him. However, when Maurice was killed by Phocas, Khosrow II probably sought to take advantage of the situation as he could use his relationship with the Byzantines as leverage to put it under his control rather continuing to submit to Roman control as it had been the case in the 590s.
Khosrow II was playing the game of geopolitics just as Maurice had. The difference is that Maurice was focusing on the West and did not want to continue antagonizing the Sassanids. He also sought to get the Armenians and Georgians under his influence. Maurice was attempting to reinforce what Justinian had built in the earlier years of the century.
Khosrow II was more opportunistic than Maurice and he wasn’t really an ally of the Iranian’s greatest rival, the Romans. He was more of a colleague of Maurice rather than a collaborator of the Byzantines. He was still a shahanshah of the Iranians and his was a great and respected empire. Imperial considerations for the Iranians were more important than submission to Rome.
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