Why did the Byzantine-Sasanian War of 602-628 happen?

The Byzantine-Sasanian War of 602-628 was the final and most devastating conflict between the Byzantine and Sasanian empires. Triggered by the murder of the Byzantine emperor Maurice, Sasanian king Khosrow II launched a massive invasion that eventually exhausted both superpowers. This mutual depletion left both empires critically vulnerable to the rapid expansion of the Rashidun Caliphate in the following decades.

What we know

• The war was initiated by Sasanian king Khosrow II ostensibly to avenge the murder of the deposed Byzantine emperor Maurice by Phocas in 602, ending a period of peace established after Maurice helped Khosrow regain his throne. [1, 2]
• Between 602 and 622, Sasanian forces gradually conquered much of the Levant, parts of Anatolia, Egypt, and Aegean islands. [2, 3]
• Emperor Heraclius launched counter-attacks from 622 to 626, forced the Persians onto the defensive, and invaded the Persian heartland after allying with Turks following the defeat of an attack on Constantinople. [1, 2]
• The exhaustion of human and material resources in both empires left them vulnerable to the emergent Islamic Rashidun Caliphate in the 630s. [2, 5]

Still open

• Historical sources vary regarding whether the Sasanian capture of Jerusalem in 614 occurred without resistance or after a siege and the breaching of the city walls with artillery.
• Sources present widely differing figures for Christian casualties during the conquest of Jerusalem, ranging from the low thousands to tens of thousands or higher.

Where to go next

• whoWho was Emperor Heraclius?
• whenWhat happened at the Battle of Nineveh in 627?
• a case in pointHow did the Sasanian conquest of Jerusalem unfold?
• what followedWhat caused the rise of the Rashidun Caliphate?
• the bigger pictureWhat were the Roman-Persian Wars?
• an unexpected connectionHow did a war between empires lift a centuries-long ban on Jews in Jerusalem?The Sasanian invasion and alliance with Jewish rebels interrupted the long-standing Byzantine ban, allowing Jews to enter Jerusalem for the first time since 136 AD.

Sources

• [1]Byzantine–Sassanid War of 602–628 (Wikidata Q2326099) · CC0 1.0
• [2]Byzantine–Sasanian War of 602–628 · CC BY-SA 4.0
• [3]Sasanian Egypt · CC BY-SA 4.0
• [4]Sasanian conquest of Jerusalem · CC BY-SA 4.0
• [5]Sasanian Empire · CC BY-SA 4.0