Heraclius (575-641) was son of Heraclius the Elder; exarch of Africa; who led a revolt against the usurper emperor Phocas; deposing him in 610. Heraclius became emperor and was immediately forced to deal with multiple threats on many frontiers.

One of the main frontiers was the Byzantine-Sassanid War of 602-628 against King Khosrau II and the Sassanid Empire. The Sassanids managed to fight all the way to the walls of Constantinople before failing to penetrate them; allowing Heraclius to counter-attack and drive them all the way back to the Sassanid capital of Ctesiphon. Khosrau was executed by his son Kavadh II; and a peace treaty was agreed. The Sassanid Empire soon fell to the Muslim conquests; another threat Heraclius had to deal with.

Heraclius was credited for making Greek the Byzantine Empire's official language; as well as for his enlarging of the empire and his reorganisation of government and military. Though his attempts at religious harmony failed; he was successful in returning the True Cross to Jerusalem.