The Byzantine–Sasanian wars, also known as the Irano-Byzantine wars^[2] refers to a series of conflicts between the Eastern Roman (Byzantine) Empire and the Sassanian Empire of Persia. A continuation of the Roman–Persian Wars, the conflict involved several smaller campaigns and peace treaties lasting for years at a time.

The Roman Empire had reached its greatest extent under the Emperor Trajan. However, before Trajan, the Emperor Augustus set about stabilizing the frontiers of the Empire. As a result, the Romans were more interested in simply defending their territory and consolidating the Empire rather than in attempting to conquer Persia.

The Romans employed the best infantry of the time: heavily armed and armoured soldiers plus numerous auxiliaries. By the 4th century A.D., armour was less often used, and by the 5th century, Germanic mercenaries were employed. The Romans continued to use heavy infantry of the legionary type, but these were less mobile than the Persian horse archers, although more effective in attrition warfare. From the 3rd century heavy cataphract cavalry wearing full horse and rider armour became an increasingly important component of Roman armies.

The Sasanian dynasty had a more or less secure northern and eastern border due to the mountainous terrain of the Caucasus, Middle Asia and Afghanistan. The Western border was determined by Sassanid control of Mesopotamia. Due to the flat nature of the land, it was easy to overrun and difficult to defend. With these natural boundaries, the Sasanian Persians had only Westward to expand. The Sasanians were becoming as efficient in siege warfare as the Romans, capturing and sacking a number of cities as part of a larger goal of exacting tribute and land from the Romans. However, the nature of the warfare was one of attrition with heavy casualties on both sides. As a result, little land was exchanged between the two powers; rather, vassal states and tribute would have been demanded.

The Sassanid Persians employed cavalry archers and heavy cataphracts to counter the heavy Roman infantry. In battle, these archers had varying degrees of effect. Soon the Romans began to adopt similar methods of warfare.

Following Trajan's successful conquest of Mesopotamia in the 2nd century, the Parthian Empire began to decline. Ctesiphon was overrun by the Romans but the lack of any permanent establishment meant that the Sassanid dynasty filled the power vacuum in the region and started a new Persian Empire in 224. The Sasanians were a more aggressive enemy than their Parthian predecessors and consequently, the Romans found themselves fighting a more dangerous Eastern opponent at a time when the Roman Empire was weakening due to the civil chaos arising from the death of the Roman Emperor Commodus.

Persian-Roman conflict was renewed shortly after the overthrowing of the Parthian rule and the foundation of the Sasanian empire by Ardashir I (226–241), who raided in Mesopotamia and Syria in 230, demanding the restitution of the Achaemenid possessions in Europe.^[3] After fruitless negotiations, Alexander Severus set out against Ardashir in 232; one column of his army marched successfully into Armenia, while two other columns operated to the south and failed, mostly on account of physical hardship. In any case Ardashir was repulsed and Alexander Severus celebrated a triumph in Rome.^[4] In 238–240, towards the end of his reign, Ardashir attacked again, taking several cities in Syria and Mesopotamia, including Carrhae, Nisibis and Hatra.^[5]

The struggle resumed and intensified under Ardashir's successor Shapur I; he invaded Mesopotamia but his forces were defeated at a battle near Resaena in 243; Carrhae and Nisibis were retaken by the Romans.^[6] Encouraged by this success, the emperor Gordian III advanced down the Euphrates but was defeated near Ctesiphon in the Battle of Misiche in 244. Gordian either died in the battle or was murdered by his own men; Philip became emperor, and paid 500,000 denarii to the Persians.^[7]

In the early 250s, Philip was involved in a struggle over the control of Armenia. Shapur had the Armenian king murdered, and re-opened hostilities against Rome; he defeated the Roman troops at the Battle of Barbalissos, and then probably took and plundered Antioch.^[8] Some time between 258 and 260, Shapur captured the emperor Valerian after crushing his army at the Battle of Edessa, but his subsequent advance into Anatolia ended, when Odaenathus, the ruler of Palmyra, attacked detachments of the Persians, causing them to retreat to their homeland.^[9]

In 275 and 282 Aurelian and Probus respectively planned to invade Persia, but they were both murdered before they were able to fulfill their plans.^[10] In 283 the emperor Carus launched a successful invasion of Persia, sacking its capital, Ctesiphon; they would probably have extended their conquests, if Carus had not died in December of the same year.^[11] After a brief period of peace during Diocletian's early reign, Narseh renewed hostilities with the Romans invading Armenia, and defeated Galerius not far from Carrhae in 296 or 297.^[12] However, in 298 Galerius defeated Narseh at the Battle of Satala, sacked the capital Ctesiphon and captured the Persian treasury and royal harem. The Roman victory was the most decisive for many decades: many cities east of the Tigris were given to the Romans including Tigranokert, Saird, Martyropolis, Balalesa, Moxos, Daudia, and Arzan. Also, control of Armenia was given to the Romans.^[13]

The arrangements of 299 proved long-lasting. It was Shapur II who broke the long peace between the two empires in mid 330s, and mounted a series of offensives against the Romans with little lasting effect.^[12] Shapur launched a new campaign in 359, successfully laying siege to Amida, and provoked a major offensive in 363 by the Roman Emperor Julian.^[14] Despite victory^[15][16] at the Battle of Ctesiphon, Julian was unable to capture the fortified city itself. He was killed the same year at the Battle of Samarra, during a difficult retreat along the Tigris. His successor Jovian found his army in a disadvantageous position, and was forced to hand over the former Roman possessions east of the Tigris, as well as Nisiris and Singara; Armenia was also abandoned by the Romans, and was soon conquered by Shapur.^[17]

In 383 or 384 Armenia again became a bone of contention between the Roman and the Sasanian empires, but hostilities did not occur.^[18] With both empires preoccupied by barbarian threats from the north, a largely peaceful period followed, interrupted only by two brief wars, the first in 421–422 after Bahram V persecuted high-ranking Persian officials who had converted to Christianity, and the second in 440, when Yazdegerd II raided Roman Armenia.^[19]

The Anastasian War ended the longest period of peace the two powers ever enjoyed. It broke out when the Roman emperor Anastasius I refused to provide financial support to the Persian king, Kavadh I, who tried to gain the money by force.^[20] In 502 Kavadh quickly captured the unprepared city of Theodosiopolis,^[21] and then besieged the fortress-city of Amida through the autumn and winter (502–503). The siege of the city proved to be a far more difficult enterprise than Kavadh expected; the defenders repelled the Persian assaults for three months before being defeated.^[22] In 503 the Romans attempted an ultimately unsuccessful siege of the Persian-held Amida while Kavadh invaded Osroene, and laid siege to Edessa with the same results.^[23]

Finally in 504, the Romans gained the upper hand with the renewed investment of Amida, leading to the hand-over of the city. That year an armistice was agreed to as a result of an invasion of Armenia by the Huns from the Caucasus. Negotiations between the two powers took place, but such was their distrust that in 506 the Romans, suspecting treachery, seized the Persian officials. Once released, the Persians preferred to stay in Nisibis.^[24] In November, 506, a treaty was finally agreed upon, but little is known of what the terms of the treaty were. Procopius states that peace was agreed for seven years, and it is likely that some payments was made to the Persians.^[25]

In 505 Anastasius ordered the building of a great fortified city at Dara. The dilapidated fortifications were also upgraded at Edessa, Batnac and Amida.^[26] Although no further large-scale conflict took place during Anastasius' reign, tensions continued, especially while work continued at Dara. This construction project was to become a key component of the Roman defenses, and also a lasting source of controversy with the Persians, who complained that it violated the treaty of 422, by which both empires had agreed not to establish new fortifications in the frontier zone. Anastasius, however, pursued the project, and the walls were completed by 507/508.^[24]

In 524/525 Kavadh proposed to Justin I to adopt his son, Khosrau. The proposal was initially greeted with enthusiasm by the Roman emperor and his nephew, Justinian, but Justin's quaestor, Proculus, opposed the move. Despite the final breakdown of the negotiations, it was not until 530 that full-scale warfare on the main eastern frontier broke out. In the intervening years, the two sides preferred to wage war by proxy, through Arab allies in the south and Huns in the north.^[27] Tensions between the two powers were further heightened by the defection of the Iberian king Gourgen to the Romans: in 524/525 the Iberians rose in revolt against Persia, following the example of the neighboring Christian kingdom of Lazica, and the Romans recruited Huns from the north of the Caucasus to assist them.^[28]

By 526–527, overt fighting between the two empires had broken out in the Transcaucasus region and upper Mesopotamia. At the same time, the Persians continued to exert pressure on the Romans to obtain funds from them.^[29] The early years of war favored the Persians: by 527 the Iberian revolt had been crushed, a Roman offensive against Nisibis and Thebetha in that year was unsuccessful and forces trying to fortify Thannuris and Melabasa were prevented from doing so by Persian attacks.^[30] Attempting to remedy the deficiencies revealed by these Persian successes, the new Roman emperor, Justinian I, reorganized the eastern armies.^[31] In 528 Belisarius tried unsuccessfully to protect Roman workers in Thannuris, undertaking the construction of a fort right on the frontier.^[32] Damaging raids on Syria by the Lakhmids in 529 encouraged Justinian to strengthen his own Arab allies, helping the Ghassanid leader Al-Harith ibn Jabalah turn a loose coalition into a coherent kingdom.

In 530 the Romans defeated the Persian troops at Dara and Satala. In 531 Belisarius was defeated by Persian and Lakhmid forces at the Battle of Callinicum, but, during the summer of the same year, the Romans captured some forts in Armenia, and effectively repulsed the Persian offensive.^[33] Immediately after the Roman failure at Callinicum, which resulted in the dismissal of Belisarius, unsuccessful negotiations between Justinian's envoy, Hermogenes, and Kavadh took place.^[34] Justinian then took steps to bolster the Roman position, trying, at the same time, to engage Kavadh diplomatically. In spring 532 negotiations re-opened between the Roman envoys and the new Persian king, Khosrau I, who needed to secure his own position. The two sides finally came to an agreement, and the Eternal Peace, which lasted less than eight years, was signed in September 532. Both powers agreed to return all occupied territories and the Romans to make a one-off payment of 110 centenaria (11,000 lbs of gold). The Romans recovered the Lazic forts, and Iberia remained in Persian hands, but the Iberians who had left their country were allowed to either remain in Roman territory or return to their native land.^[35]

The successful campaigns of Belisarius in the west encouraged the Persians to return to war, both taking advantage of Roman preoccupation elsewhere and seeking to check the expansion of Roman territory and resources.^[36] In 539 the resumption of hostilities was foreshadowed by a Lakhmid raid led by al-Mundhir IV, which was defeated by the Ghassanids under al-Harith ibn Jabalah. In 540, the Persians broke the "Treaty of Eternal Peace" and Khosrau I invaded Syria, destroying the great city of Antioch and deporting its population to Weh Antiok Khosrow in Persia; as he withdrew, he extorted large sums of money from the cities of Syria and Mesopotamia and systematically looted the key cities. In 541 he invaded Lazica in the north.^[37]

Belisarius was quickly recalled by Justinian to the East to deal with the Persian threat, while the Ostrogoths in Italy, who were in touch with the Persian King, launched a counter-attack under Totila. Belisarius took the field and waged an inconclusive campaign against Nisibis in 541. In the same year Lazica switched its allegiance to Persia and Khosrau led an army to secure the kingdom. In 542 Khosrau launched another offensive in Mesopotamia, and unsuccessfully attempted to capture Sergiopolis.^[38] He soon withdrew in the face of an army under Belisarius, en route sacking the city of Callinicum.^[39] Attacks on a number of Roman cities were repulsed and the Persian general Mihr-Mihroe was defeated and captured at Dara by John Troglita.^[40]

(The war dragged on under other generals and was to some extent hindered by the so-called Plague of Justinian, because of which Khosrau temporarily withdrew from Roman territory^[41])

In 543, the Romans fielded a force of 30,000 troops commanded by the magister militum of the East, Martin, for an invasion of Persian-controlled Armenia. They launched an offensive against Dvin, but were defeated by a small Persian force at Anglon. In 544 Khosrau besieged Edessa without success and was eventually bought off by the defenders. The Edessenes paid five centenaria to Khosrau, and the Persians departed after nearly two months.^[42] In the wake of the Persian retreat, two Roman envoys, the newly appointed magister militum, Constantinus, and Sergius proceeded to Ctesiphon to arrange a truce with Khosrau.^[43] A five-year truce was agreed in 545, secured by Roman payments to the Persians.^[44]

In early 548 AD, king Gubaz of Lazica, having found Persian protection oppressive, asked Justinian to restore the Roman protectorate. The emperor seized the chance, and in 548/549 AD combined Roman and Lazic forces under Gubaz and the magister militum of Armenia Dagistheus won a series of victories against Persian armies under Mihr-Mihroe and Khorianes, but failed in repeated attempts to take the fort of Petra (present-day Tsikhisdziri, north of Batumi).^[45] In 551 AD, general Bassas who replaced Dagistheus put Abasgia and the rest of Lazica under control, and finally subjected Petra, demolishing its fortifications.^[46]

In the same year, however, a Persian army under Mihr-Mihroe but was defeated with heavy losses.^[47] That year the truce which had been established in 545 AD was renewed outside Lazica for a further five years, with the Romans paying 2,000 lbs of gold each year.^[48] The Romans failed to completely expel the Sasanian from Lazica, and in 554 AD Mihr-Mihroe launched a new attack, and captured the fortress of Telephis, which was commanded by general Martin.^[47]

In 557 AD Khusro, who had now to deal with the White Huns, dispatched his envoy Izedh Gushnap to Constantinople and renewed the truce, this time without excluding Lazica, where they kept only a toehold; negotiations continued for a definite peace treaty.^[49] Finally, in 561 AD, Justinian's envoy, Peter the Patrician, and Izedh Gushnap put together a 50-year peace. The Persians agreed to evacuate Lazica, and received an annual subsidy of 30,000 nomismata annually. Both sides agreed not to build new fortifications near the frontier and to ease restrictions on diplomacy and trade between the two empires.^[50]

The war began, when the Armenians revolted against Sasanian rule in early 572 AD. Justin II took them under his protection, and sent his nephew, Marcian, against the Persians. Roman troops raided Arzanene, and invaded Persian Mesopotamia, defeating its local forces.^[51] Marcian's sudden dismissal, however, and the arrival of troops under Khosrau resulted in the ravaging of Syria, the failure of the Roman siege of Nisibis, and the fall of Dara.^[52] At a cost of 45,000 solidi a one-year truce (later in the year extended to five years^[53]) was arranged, though the Persians still wanted to restore control in Armenia.^[54]

In 576 AD Khosrau I attempted to combine aggression in Armenia with discussion of a permanent peace. He failed however to take Theodosiopolis, and after a confrontation near Melitene the Persian royal baggage was captured; there were severe Persian losses either in set battle or during a disorganized fleet over the Euphrates.^[55] The Romans exploited Persian disarray by invading deep into Persian territory, raiding Atropatene.^[55] Persian confidence revived, when Tamkhusro defeated Justinian in Persian Armenia, where Roman actions had alienated local inhabitants.^[56] In the spring of 578 AD the Persians raided Byzantine Mesopotamia; the Roman general Maurice retaliated by invading Arzanene; he also took and garrisoned the stronghold of Aphumon,and sacked Singara in Persian Mesopotamia. Khosrau I died early the next year, defeated after so many victories.^[57]

In 580, Hormizd IV abolished the Caucasian Iberian monarchy, and turned Iberia into a Persian province ruled by a marzpan (governor).^[58][59] During the 580s the war continued in inconclusive fashion, with victories on both sides. In 582 AD Maurice defeated Tamkhusro, who was killed, but the Roman general did not follow up his victory; he had to hurry to Constantinople to pursue his imperial ambitions.^[60] In 589 AD the Persians achieved a last success, capturing Martyropolis through treachery, but in the same year the stalemate was shattered when the Persian general Bahram Chobin, having been dismissed and humiliated by Hormizd IV, raised a rebellion. Hormizd was overthrown in a palace coup in 590 AD and replaced by his son Khosrau II, but Bahram pressed on with his revolt regardless and the defeated Khosrau was soon forced to flee for safety to Roman territory, while Bahram took the throne as Bahram VI. With support from Maurice, Khosrau raised a rebellion against Bahram, and in 591 AD the combined forces of his supporters and the Romans defeated Bahram, restoring Khosrau II to power and bringing the war to an end. In exchange for their help, Khosrau not only returned Dara and Martyropolis but also agreed to cede the western half of Iberia and more than half of Persian Armenia to the Romans.^[61]

During Maurice's Balkan campaigns, he and his family were murdered by Phocas in November 602 after a mutiny. Khosrau II seized this opportunity to attack the Roman Empire and reconquer the Roman province of Mesopotamia.^[62] The war initially went the Persians' way, partly because of Phocas' brutal repression and the succession crisis that ensued as the general Heraclius sent his nephew Nicetas to attack Egypt, enabling his son Heraclius the younger to claim the throne in 610. Phocas, an unpopular ruler who is invariably described in Byzantine sources as a "tyrant", was eventually deposed by Heraclius, who sailed to Constantinople from Carthage with an icon affixed to the prow of his ship.^[63]

By this time the Persians had conquered Mesopotamia and the Caucasus, and in 611 they overran Syria and entered Anatolia. A major counter-attack led by Heraclius two years later was decisively defeated outside Antioch by Shahrbaraz and Shahin and the Roman position collapsed; the Persians devastated parts of Asia Minor, and captured Chalcedon on the Bosporus.^[64] Over the following decade the Persians were able to conquer Palestine and Egypt (by mid-621 the whole province was in their hands^[65]) and to devastate Anatolia,^[66] while the Avars and Slavs took advantage of the situation to overrun the Balkans, bringing the Roman Empire to the brink of destruction. The Persians continued with their successful offensive, capturing Ancyra, an important military base in central Anatolia, in 620 or 622. Rhodes and several other islands in the eastern Aegean fell in 622/3, threatening a naval assault on Constantinople.^{[67][68][69][70]} Such was the despair in Constantinople that Heraclius considered moving the government to Carthage in Africa.^[71]

During these years, Heraclius strove to rebuild his army, slashing non-military expenditure, devaluing the currency and melting down Church plate, with the backing of Patriarch Sergius, to raise the necessary funds to continue the war.^[72]

On April 5 622, Heraclius left Constantinople, entrusting the city to Sergius and general Bonus as regents of his son. He assembled his forces in Asia Minor, probably in Bithynia, and, after he revived their broken morale, he launched a new counter-offensive, which took on the character of a holy war; an acheiropoietos image of Christ was carried as a military standard.^[73] The Roman army proceeded to Armenia, inflicted a defeat on an army led by a Persian-allied Arab chief, and then won a victory over the Persians under Shahrbaraz.^[74] On 25 March 624, Heraclius again left Constantinople, with his wife Martina and his two children; after he celebrated Easter in Nicomedia on April 15, he campaigned in the Caucasus, winning a series of victories in Azerbaijan and Armenia against Khosrau and his generals Shahrbaraz, Shahin and Shahraplakan.^[75] In 626 the Avars, Slavs and a Persian army commanded by Shahrbaraz besieged Constantinople, but the it ended in failure (the victory was attributed to the icons of the Virgin which were led in procession by Sergius about the walls of the city^[76]), while a second Persian army under Shahin suffered another crushing defeat at the hands of Heraclius' brother Theodore.

With the Persian war effort disintegrating, Heraclius was able to bring the Gokturks of the Western Turkic Khaganate into the war against the Sasanian in the Caucasus (see Third Perso-Turkic War). Late in 627 he launched a winter offensive into Mesopotamia, where, despite the desertion of his Turkish allies, he defeated the Persians under Rhahzadh at the Battle of Nineveh. Continuing south along the Tigris, he sacked Khosrau's great palace at Dastagird and was only prevented from attacking Ctesiphon by the destruction of the bridges on the Nahrawan Canal. Discredited by this series of disasters, Khosrau was overthrown and killed in a coup led by his son Kavadh II, who at once sued for peace, agreeing to withdraw from all occupied territories. In 629 Heraclius restored the True Cross to Jerusalem in a majestic ceremony.^[77]

The devastating impact of this last war, added to the cumulative effects of a century of almost continuous conflict, left both empires crippled. When Kavadh II died only months after coming to the throne, Persia was plunged into several years of dynastic turmoil and civil war. The Sasanians were further weakened by economic decline, heavy taxation from Khosrau II's campaigns, religious unrest, rigid social stratification, and the increasing power of the provincial landholders. The Roman Empire was even more severely affected, with its financial reserves exhausted by the war, the Balkans now largely in the hands of the Slavs, Anatolia devastated by repeated Persian invasions, and the empire's hold on Syria, Mesopotamia, Palestine and Egypt loosened by many years of Persian occupation. Neither empire was given any chance to recover, as within a few years they were struck by the onslaught of the Arabs, newly united by Islam. The Sasanian Empire rapidly succumbed to these attacks and was completely destroyed. During the Byzantine-Arab Wars, the exhausted Roman Empire's recently regained southern provinces were also lost during the Muslim conquest of Syria, Egypt and North Africa, reducing the empire to a territorial rump consisting of Anatolia and a scatter of islands and footholds in the Balkans and Italy. These remaining lands were thoroughly impoverished by frequent attacks, marking the transition from classical urban civilization to a more rural, medieval form of society. However, unlike Persia the Roman Empire (in its medieval form usually termed the Byzantine Empire) ultimately survived the Arab assault, holding onto its residual territories and repulsing two Arab sieges of its capital Constantinople in 674 and 718.