Byzantine Empire and Persia face Islam, 603-35

The initial support that Phocas received from Constantinople's citizenry faded. Phocas was a disaster for the city. He is described as having responded to all problems with little more than brutality and as alienating many.

The murders of Maurice and his family were also a disaster for relations with Persia. Khosrau II, disturbed by the death of his friend Maurice and his family, moved to avenge those deaths – or at least he used the murders as a pretext. With Khosrau's eastern borders secure, in 603 he confidently declared war against Phocas and began invading Constantinople's empire and defeating Phocas' forces. It was the beginning of twenty-six years of renewed warfare between Constantinople and the Sassanids. Khosrau II rallied his empire, claiming his right to reconstitute the great empire of the Achaemenian kings – Cyrus and Darius.

The Zoroastrian priesthood was pleased. As they saw it, their king was responsible for conquering the world in order to spread peace and the Zoroastrian faith along with individual salvation and to prepare all humankind for the great, worldwide battle against Satan at Armageddon.

Khosrau's armies occupied Syria, Palestine and Cappadocia in the middle of Asia Minor. With Constantinople weakened by renewed war against the Sassanids, the Avars joined in the advance against Constantinople and overran Thrace and beyond to Illyricum by the Adriatic Sea, seizing agricultural lands without resistance. And they were joined by the Slavs.

North Africa was the part of Constantinople's empire left untouched, and, after seven years of rule by Phocas at Constantinople, a force from Egypt led by the military-governor to Egypt, Heraclius, sailed to Constantinople intending on overthrowing him. Heraclius and his group arrived at Constantinople in 610, and, with Phocas having lost much of his support, Heraclius easily defeated him. That same year Phocas was executed on the scaffold, and Heraclius became emperor.

The War Proves Costly and Futile

Perhaps the war turned Khosrau II against Christianity – the faith of what again had become an enemy empire. Whatever sympathies Khosrau had had toward Christianity and the many Christians within his empire early in his reign, in his later years he showered favor upon the Zoroastrians. He built fire temples for them and he sanctioned their persecution of Christians.

In 614, Khosrau's forces sacked Jerusalem, massacring 90,000 Christians, burning to the ground many Christian churches and carrying Christian relics back to Persia. In 614, the Avars sacked cities in Greece. In 616, Khosrau's forces invaded and occupied Egypt, meeting little resistance. Then in 617 the Avars neared Constantinople, while large numbers of Slavs were spreading southward and settling in Greece. In 623, Slavs ravaged the island of Crete. In 626, Avars, supported by Slavs, attacked the walls of Constantinople. A Sassanid force also assaulted the city. The Patriarch of Constantinople, Sergius, led a courageous defense of Constantinople and defeated the Avars. The Avars withdrew to Pannonia and never again threatened Constantinople. Unable to penetrate Constantinople's walls and facing Constantinople's superior navy, Khosrau II withdrew his forces from around the city.

The Sassanids had overextended their forces. Their victorious move into Egypt, Palestine and Asia Minor proved hollow as they had too few people to occupy these areas while holding off a counterattack by Constantinople, drawing on the manpower of its empire. With his superior navy, Emperor Heraclius sailed into the Black Sea, his troops disembarking behind Persia's armies. Heraclius' troops began marching toward the Sassanid capital, Ctesiphon, destroying what they could along the way, while the Persians fled before them. They broke dikes to create floods in order to slow Heraclius' progress. They destroyed the great canal works in Mesopotamia, which were to fill with silt and remain neglected.

Khosrau II fled Ctesiphon. His armies remained undefeated and angry in their humiliation. Khosrau found a scapegoat for his defeat in the commander of his armies, Shahrbaraz. He planned to execute Shahrbaraz, but Khosrau's generals, who had often smarted from his insults, joined with the old rivals of the monarchy, the nobles, and imprisoned Khosrau. They fed Khosrau bread and water and killed eighteen of his sons before his eyes. Then the generals, encouraged by his remaining son, Sheroye, executed Khosrau.

Sheroye was crowned king, and he took the name Kavadh II. In 630, Kavadh signed a peace treaty with Constantinople that returned Egypt, Palestine, Asia Minor and western Mesopotamia to Constantinople. Kavadh returned to Jerusalem relics that had been taken from there, including what were believed to be the remains of the cross of Jesus. People of both empires rejoiced at the end of a long war that had bled the empires for so many years. Khavadh agreed to withdraw his troops from Egypt. Prisoners of war were to be exchanged, and the two sides recognized the boundaries that had existed before the war. The war had gained nothing for either side.

Heraclius personally replaced the "True Cross" on its shrine in Jerusalem. He did not notice that on that very day some Arabs attacked Constantinople's garrison near the river Jordan. These Arabs were Islamists, beginning their assaults on territory nominally part of Constantinople's empire.

After less than a year as emperor, Kavadh died. His seven-year-old son, Ardashir III, succeeded him, Ardashir ruling in name only until general Shahrbaraz killed the boy and usurped the throne. In turn, Shahrbaraz' own soldiers killed him and dragged his body through the streets of Ctesiphon. Anarchy swept through the Sassanid empire, already exhausted by twenty-six years of war. In the coming four years, nine men tried to gain the throne, and all disappeared through flight, assassination or death by disease. Cities and provinces declared their independence.

The Sassanid empire was fading, and Constantinople was weakened, while a neighboring force that would expand against the two – Islam – was on the rise.