The Byzantine–Sasanian War of 602–628, also called the Last Great War of Antiquity, was fought between the Byzantine Empire and the Sasanian Empire. It was the final and most devastating conflict of the Roman–Persian wars (54 BC – AD 628). The previous war between the two powers had ended in 591 after the emperor Maurice helped the Sasanian king Khosrow II regain his throne. In 602, Maurice was murdered by his political rival Phocas. Khosrow declared war, ostensibly to avenge the death of the deposed emperor Maurice. This became a decades-long conflict, the longest war in the series, and was fought throughout the Middle East, the Aegean Sea, and before the walls of Constantinople itself.
From 602 to 622, the Sassanians gradually conquered much of the Levant, parts of Anatolia, and—for the first time—Egypt and several islands in the Aegean Sea. The counter-attacks of the new Byzantine emperor Heraclius from 622 to 626 eventually forced the Persians onto the defensive. Allied with the Avars and Slavs, the Persians attempted to take Constantinople in 626, but were defeated. In 627, allied with Turks, Heraclius invaded the heartland of Persia. After the Battle of Nineveh (627), Iranian forces were finally broken, forcing civil war-torn Persia to seek peace.
By the end of the war, both sides had exhausted their human and material resources. They were thus vulnerable to the emergence of the Islamic Rashidun Caliphate in the 630s, whose forces invaded both empires. Over the course of the rest of the 7th century, Muslim armies swiftly conquered the Levant, Mesopotamia, Persia, the Caucasus, Egypt, North Africa, and the Iberian Peninsula alongside southern parts of France after 711 AD. These conquests led to the fall of the Sasanian Empire and a significant reduction in the size and power of the Byzantine Empire, which over the following centuries would fight several wars with the Muslim powers for control of the Near East.
After decades of inconclusive fighting, Emperor Maurice ended the Byzantine–Sasanian War of 572–591 by helping the exiled Sasanian prince Khosrow, the future Khosrow II, to regain his throne from the usurper Bahram Chobin. In return, the Sasanians ceded parts of northeastern Mesopotamia, much of Persian Armenia and Caucasian Iberia to the Byzantines, though the exact details are not clear. More importantly for the Byzantine economy, they no longer had to pay tribute to the Persians. Emperor Maurice then began new campaigns in the Balkans to stop incursions by the Slavs and Avars.
The generosity and campaigns of emperor Tiberius II had eliminated the surplus in the treasury left from the time of Justin II. In order to generate a reserve in the treasury, Maurice instituted strict fiscal measures and cut army pay; which led to four mutinies. The final mutiny in 602 resulted from Maurice ordering his troops in the Balkans to live off the land during the winter. The army proclaimed Phocas, a Thracian centurion, as emperor. Maurice attempted to defend Constantinople by arming the Blues and the Greens – supporters of the two major chariot racing teams of the Hippodrome – but they proved ineffective. Maurice fled but was soon intercepted and killed by the soldiers of Phocas.
Upon the murder of Maurice, Narses, governor of the Byzantine province of Mesopotamia, rebelled against Phocas and seized Edessa, a major city of the province. Emperor Phocas instructed general Germanus to besiege Edessa, prompting Narses to request help from the Persian king Khosrow II. Khosrow, who was only too willing to help avenge Maurice, his "friend and father-[in-law]", used Maurice's death as an excuse to attack the Byzantine Empire, trying to reconquer Armenia and Mesopotamia.
Germanus died in battle against the Persians. An army sent by Phocas against Khosrow was defeated near Dara in Upper Mesopotamia, leading to the capture of that important fortress in 605. Narses escaped from Leontius, the eunuch appointed by Phocas to deal with him, but when Narses attempted to return to Constantinople to discuss peace terms, Phocas ordered him seized and burned alive. The death of Narses along with the failure to stop the Persians damaged the prestige of Phocas's military regime.
In 608, general Heraclius the Elder, Exarch of Africa, revolted, urged on by Priscus, the Count of the Excubitors and son-in-law of Phocas. Heraclius proclaimed himself and his namesake son as consuls – thereby implicitly claiming the imperial title – and minted coins with the two wearing the consular robes.
At about the same time rebellions began in Roman Syria and Palaestina Prima in the wake of Heraclius's revolt. In 609 or 610 the Patriarch of Antioch, Anastasius II, died. Many sources claim that the Jews were involved in the fighting, though it is unclear where they were members of factions and where they were opponents of Christians. Phocas responded by appointing Bonus as comes Orientis (Count of the East) to stop the violence. Bonus punished the Greens, a horse racing party, in Antioch for their role in the violence in 609.
Heraclius the Elder sent his nephew Nicetas to attack Egypt. Bonus went to Egypt to try to stop Nicetas, but was defeated by the latter outside Alexandria. In 610, Nicetas succeeded in capturing the province, establishing a power base there with the help of Patriarch John the Almsgiver, who was elected with the help of Nicetas.
The main rebel force was employed in a naval invasion of Constantinople, led by the younger Heraclius, who was to be the new emperor. Organized resistance against Heraclius soon collapsed, and Phocas was handed to him by the patrician Probos (Photius). Phocas was executed, though not before a celebrated exchange of comments between him and his successor:"Is it thus", asked Heraclius, "that you have governed the Empire?""Will you," replied Phocas, with unexpected spirit, "govern it any better?"
The elder Heraclius disappears soon afterward from sources, supposedly dying, though the date is unknown.
After marrying his betrothed Fabia Eudokia and being crowned by the Patriarch, the 35-year-old Heraclius set out to perform his work as emperor. Phocas's brother Comentiolus commanded a sizable force in central Anatolia but was assassinated by the Armenian commander Justin, removing a major threat to Heraclius's reign. Still, transfer of the forces commanded by Comentiolus had been delayed, allowing the Persians to advance further in Anatolia. Trying to increase revenues and reduce costs, Heraclius limited the number of state-sponsored personnel of the Church in Constantinople by not paying new staff from the imperial fisc. He used ceremonies to legitimize his dynasty, and he secured a reputation for justice to strengthen his grip on power.
The Persians took advantage of this civil war in the Byzantine empire by conquering frontier towns in Armenia and Upper Mesopotamia. Along the Euphrates, in 609, they conquered Mardin and Amida (Diyarbakır). Edessa, which some Christians are said to have believed would be defended by Jesus himself on behalf of King Abgar V of Edessa against all enemies, fell in 610.
In Armenia, the strategically important city of Theodosiopolis (Erzurum) surrendered in 609 or 610 to Ashtat Yeztayar, because of the persuasion of a man who claimed to be Theodosius, the eldest son and co-emperor of Maurice, who had supposedly fled to the protection of Khosrow. In 608, the Persians under general Shahin launched a raid into Anatolia that reached Chalcedon, across the Bosporus from Constantinople. The Persian conquest was a gradual process; by the time of Heraclius's accession the Persians had conquered all Roman cities east of the Euphrates and in Armenia before moving on to Cappadocia, where Shahin took Caesarea Mazaca. There, Phocas's son-in-law Priscus, who had encouraged Heraclius and his father to rebel, started a year-long siege to trap them inside the city.
Heraclius's accession as Emperor did little to reduce the Persian threat. Heraclius began his reign by attempting to make peace with the Persians, since Phocas, whose actions were the original casus belli, had been overthrown. The Persians rejected these overtures, however, since their armies were widely victorious. According to historian Walter Kaegi, it is conceivable that the Persians' goal was to restore or even surpass the boundaries of the Achaemenid Empire by destroying the Byzantine empire, though because of the loss of the royal Persian archives, no document survives to conclusively prove this.