Byzantine Empire: Heraclian Dynasty

Even though the Empire had gained smaller successes over the Slavs and Avars in pitched battles across the Danube, both enthusiasm for the army and faith in the government had lessened considerably. Unrest had reared its head in Byzantine cities as social and religious differences manifested themselves into Blue and Green factions that fought each other in the streets. The final blow to the government was a decision to cut the pay of its army in response to financial strains. The combined effect of an army revolt led by a junior officer named Phocas and major uprisings by the Greens and Blues forced Maurice to abdicate. The Senate approved Phocas as the new Emperor and Maurice, the last emperor of the Justinian Dynasty, was murdered along with his four sons.

The Persian King Khosrau II responded by launching an assault on the Empire, ostensibly to avenge Maurice, who had earlier helped him to regain his throne. Phocas was already alienating his supporters with his repressive rule (introducing torture on a large scale), and the Persians were able to capture Syria and Mesopotamia by 607. By 608, the Persians were camped outside Chalcedon, within sight of the imperial capital of Constantinople, while Anatolia was ravaged by Persian raids. Making matters worse was the advance of the Avars and Slavic tribes heading south across the Danube and into Imperial territory.

While the Persians were making headway in their conquest of the eastern provinces, Phocas chose to divide his subjects rather than unite them against the threat of the Persians. Perhaps seeing his defeats as divine retribution, Phocas initiated a savage and bloody campaign to forcibly convert the Jews to Christianity. Persecutions and alienation of the Jews, a frontline people in the war against the Persians helped drive them into aiding the Persian conquerors. As Jews and Christians began tearing each other apart, some fled the butchery into Persian territory. Meanwhile, it appears that the disasters befalling the Empire led the Emperor into a state of paranoia — although it must be said that there were numerous plots against his rule and execution followed execution.

The Byzantine–Sasanian War of 602–628 was the final and most devastating of the series of wars fought between the Byzantine Empire and the Sasanian Empire of Iran. This became a decades-long conflict, the longest war in the series, and was fought throughout the Middle East: in Egypt, the Levant, Mesopotamia, the Caucasus, Anatolia, Armenia, the Aegean Sea and before the walls of Constantinople itself. While the Persians proved largely successful during the first stage of the war from 602 to 622, conquering much of the Levant, Egypt, several islands in the Aegean Sea and parts of Anatolia, the ascendancy of the emperor Heraclius in 610 led, despite initial setbacks, to a status quo ante bellum. Heraclius' campaigns in Iranian lands from 622 to 626 forced the Persians onto the defensive, allowing his forces to regain momentum. Allied with the Avars and Slavs, the Persians made a final attempt to take Constantinople in 626, but were defeated there. In 627, allied with Turks, Heraclius invaded the heartland of Persia.

Due to the overwhelming crisis facing the Empire that had pitched it into chaos, Heraclius the Younger now attempted to seize power from Phocas in an effort to better Byzantium's fortunes. As the Empire was led into anarchy, the Exarchate of Carthage remained relatively out of reach of Persian conquest. Far from the incompetent Imperial authority of the time, Heraclius, the Exarch of Carthage, with his brother Gregorius, began building up his forces to assault Constantinople.

After cutting off the grain supply to the capital from his territory, Heraclius led a substantial army and a fleet in 608 to restore order in the Empire. Heraclius gave the command of the army to Gregorius' son, Nicetas, whilst command of the fleet went to Heraclius' son, Heraclius the Younger. Nicetas took part of the fleet and his forces to Egypt, seizing Alexandria towards the end of 608. Meanwhile, Heraclius the Younger headed to Thessalonica, from where, after receiving more supplies and troops, he sailed for Constantinople. He reached his destination on 3 October 610, where he was unopposed as he landed off the shores of Constantinople, citizens greeting him as their deliverer.

The reign of Phocas officially ended in his execution and the crowning of Heraclius by the Patriarch of Constantinople two days later on 5 October. A statue of Phocas that rested in the Hippodrome was pulled down and set aflame, along with the colors of the Blues that supported Phocas.

One of the most important legacies of Heraclius was changing the official language of the Empire from Latin to Greek.

In 613, the Byzantine army led by Emperor Heraclius suffered a crushing defeat at Antioch against a Persian Sassanid army under Generals (spahbed) Shahin and Shahrbaraz . This allowed the Persians to move freely and swiftly in all directions. This surge caused the cities of Damascus and Tarsus to fall, along with Armenia. More seriously, however, was the loss of Jerusalem, which was besieged and captured by the Persians in three weeks. Countless churches in the city (including the Holy Sepulchre) were burnt and numerous relics, including the True Cross, the Holy Lance and the Holy Sponge, present at the time of Jesus Christ's death, were now in Ctesiphon, the Persian capital. The Persians remained poised outside of Chalcedon, not too far from the capital, and the province of Syria was in total chaos.

In 615, during the ongoing war with the Byzantine Empire, the Sasanian army under spahbod Shahin invaded Asia Minor and reached Chalcedon, across the Bosporus from Constantinople. It was at this point, according to Sebeos, that Heraclius had agreed to stand down and was about ready to become a client of the Sasanian emperor Khosrow II, allowing the Roman Empire to become a Persian client state, as well as even allow Khosrow II to choose the emperor. The Sassanids had already captured Roman Syria and Palestine in the previous year. After negotiations with Byzantine Emperor Heraclius, a Byzantine ambassador was sent to Persian Shahanshah Khosrau II, and Shahin withdrew again to Syria.

The Sasanian conquest of Egypt took place between 618 and 621, when the Sasanian Persian army defeated the Byzantine forces in Egypt and occupied the province. The fall of Alexandria, the capital of Roman Egypt, marked the first and most important stage in the Sasanian campaign to conquer this rich province, which eventually fell completely under Persian rule within a couple of years.

The Heraclius' campaign of 622, erroneously also known as the Battle of Issus, was a major campaign in the Byzantine–Sassanid War of 602–628 by emperor Heraclius that culminated in a crushing Byzantine victory in Anatolia.

In 622, the Byzantine emperor Heraclius, was ready to mount a counter-offensive against the Sassanid Persians who had overrun most of the eastern provinces of the Byzantine Empire. Heraclius won a crushing victory over Shahrbaraz somewhere in Cappadocia. The key factor was Heraclius' discovery of hidden Persian forces in ambush and responding to this ambush by feigning retreat during the battle. The Persians left their cover to chase the Byzantines, whereupon Heraclius' elite Optimatoi assaulted the chasing Persians, causing them to flee.

While the Byzantines were occupied with the Persians, the Avars and Slavs poured into the Balkans, capturing several Byzantine cities. Because of the need to defend against these incursions, the Byzantines could not afford to use all their forces against the Persians. Heraclius sent an envoy to the Avar Khagan, saying that the Byzantines would pay a tribute in return for the Avars withdrawing north of the Danube. The Khagan replied by asking for a meeting on 5 June 623, at Heraclea in Thrace, where the Avar army was located; Heraclius agreed to this meeting, coming with his royal court. The Khagan, however, put horsemen en route to Heraclea to ambush and capture Heraclius, so they could hold him for ransom.

Heraclius was fortunately warned in time and managed to escape, chased by the Avars all the way to Constantinople. However, many members of his court, as well as an alleged 70,000 Thracian peasants who came to see their Emperor, were captured and killed by the Khagan's men. Despite this treachery, Heraclius was forced to give the Avars a subsidy of 200,000 solidi along with his illegitimate son John Athalarichos, his nephew Stephen, and the illegitimate son of the patrician Bonus as hostages in return for peace. This left him more able to focus his war effort completely on the Persians.

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 against three Persian armies in Armenia against Khosrow and his generals Shahrbaraz, Shahin, and Shahraplakan.;

The Battle of Sarus was a battle fought in April 625 between the Eastern Roman (Byzantine) army, led by Emperor Heraclius, and the Persian general Shahrbaraz. After a series of maneuvers, the Byzantine army under Heraclius, which in the previous year had invaded Persia, caught up with Shahrbaraz's army, which was heading towards the Byzantine capital, Constantinople, where his forces would take part in its siege together with the Avars. The battle ended in a nominal victory for the Byzantines, but Shahrbaraz withdrew in good order, and was able to continue his advance through Asia Minor towards Constantinople.

During the siege of Constantinople, Heraclius formed an alliance with people Byzantine sources called the "Khazars", under Ziebel, now generally identified as the Western Turkic Khaganate of the Göktürks, led by Tong Yabghu, plying him with wondrous gifts and the promise of marriage to the porphyrogenita Eudoxia Epiphania. Earlier, in 568, the Turks under Istämi had turned to Byzantium when their relations with Iran soured over commerce issues. Istämi sent an embassy led by the Sogdian diplomat Maniah directly to Constantinople, which arrived in 568 and offered not only silk as a gift to Justin II, but also proposed an alliance against Sasanian Iran. Justin II agreed and sent an embassy to the Turkic Khaganate, ensuring the direct Chinese silk trade desired by the Sogdians.

In the East, in 625 CE, the Turks took advantage of the Sasanian weakness to occupy Bactria and Afghanistan as far as the Indus, and establish the Yabghus of Tokharistan. The Turks, based in the Caucasus, responded to the alliance by sending 40,000 of their men to ravage the Iranian Empire in 626, marking the start of the Third Perso-Turkic War. Joint Byzantine and Göktürk operations were then focused on besieging Tiflis, where the Byzantines used traction trebuchets to breach the walls, one of the first known uses by the Byzantines. Khosrow sent 1,000 cavalry under Shahraplakan to reinforce the city, but it nevertheless fell, probably in late 628.

The siege of Constantinople in 626 by the Sassanid Persians and Avars, aided by large numbers of allied Slavs, ended in a strategic victory for the Byzantines. The failure of the siege saved the empire from collapse, and, combined with other victories achieved by Emperor Heraclius (r. 610–641) the previous year and in 627, enabled Byzantium to regain its territories and end the destructive Roman–Persian Wars by enforcing a treaty with borders status quo c. 590.

The Battle of Nineveh was the climactic battle of the Byzantine-Sassanid War of 602–628. In mid-September 627, Heraclius invaded Sasanian Mesopatamia in a surprising, risky winter campaign. Khosrow II appointed Rhahzadh as the commander of an army to confront him. Heraclius' Göktürk allies quickly deserted, while Rhahzadh's reinforcements did not arrive in time. In the ensuing battle, Rhahzadh was slain and the remaining Sasanians retreated.

Continuing south along the Tigris he sacked Khosrow'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, Khosrow was overthrown and killed in a coup led by his son Kavad II, who at once sued for peace, agreeing to withdraw from all occupied territories. The Sassanian civil war significantly weakened the Sassanian Empire, contributing to the Islamic conquest of Persia.

The last of the Roman–Persian Wars ended in 628, after Heraclius concluded a successful campaign against the Persians in Mesopotamia. At the same time, Muhammad united the Arabs under the banner of Islam. After his death in 632, Abu Bakr succeeded him as the first Rashidun Caliph. Suppressing several internal revolts, Abu Bakr sought to expand the empire beyond the confines of the Arabian Peninsula.

The Muslim conquest of the Levant occurred in the first half of the 7th century. This was the conquest of the region known as the Levant or Shaam, later to become the Islamic Province of Bilad al-Sham, as part of the Islamic conquests. Arab Muslim forces had appeared on the southern borders even before the death of Muhammad in 632, resulting in the Battle of Mu'tah in 629, but the real conquest began in 634 under his successors, the Rashidun Caliphs Abu Bakr and Umar ibn Khattab, with Khalid ibn al-Walid as their most important military leader.

The Battle of Ajnadayn was fought in July or August 634, in a location close to Beit Guvrin in present-day Israel; it was the first major pitched battle between the Byzantine (Roman) Empire and the army of the Arab Rashidun Caliphate. The result of the battle was a decisive Muslim victory. The details of this battle are mostly known through Muslim sources, such as the ninth-century historian al-Waqidi.

The siege of Damascus (634) lasted from 21 August to 19 September 634 before the city fell to the Rashidun Caliphate. Damascus was the first major city of the Eastern Roman Empire to fall in the Muslim conquest of Syria.

In April 634, Abu Bakr invaded the Byzantine Empire in the Levant and decisively defeated a Byzantine army at the Battle of Ajnadayn. The Muslim armies marched north and laid siege to Damascus. The city was taken after a monophysite bishop informed Khalid ibn al-Walid, the Muslim commander in chief, that it was possible to breach city walls by attacking a position only lightly defended at night. While Khalid entered the city by assault from the Eastern gate, Thomas, commander of the Byzantine garrison, negotiated a peaceful surrender at the Jabiyah gate with Abu Ubaidah, Khalid's second in command. After the surrender of the city, the commanders disputed the terms of the peace agreement.

The Battle of Fahl was a major battle in the Muslim conquest of Byzantine Syria fought by the Arab troops of the nascent Islamic caliphate and Byzantine forces at or near Pella (Fahl) and nearby Scythopolis (Beisan), both in the Jordan Valley, in December 634 or January 635. Byzantine troops smarting from their rout by the Muslims at the battle of Ajnadayn or the Yarmuk had regrouped in Pella or Scythopolis and the Muslims pursued them there. The Muslim cavalry faced difficulty traversing over the muddied grounds around Beisan as the Byzantines cut irrigation ditches to flood the area and stave off the Muslim advance. The Muslims ultimately defeated the Byzantines, who are held to have suffered enormous casualties. Pella was subsequently captured, while Beisan and nearby Tiberias capitulated after short sieges by detachments of Muslim troops.

After Abu Bakr died in 634, his successor, Umar, was determined to continue the Caliphate's expansion deeper into Syria. Though previous campaigns led by Khalid had been successful, he was replaced by Abu Ubaidah. Having secured southern Palestine, Muslim forces now advanced up the trade route, and Tiberias and Baalbek fell without much struggle and conquered Emesa early in 636. The Muslims then continued their conquest across the Levant.

To check the Arab advance and to recover lost territory, Emperor Heraclius had sent a massive expedition to the Levant in May 636. As the Byzantine army approached, the Arabs tactically withdrew from Syria and regrouped all their forces at the Yarmuk plains close to the Arabian Peninsula, where they were reinforced, and defeated the numerically superior Byzantine army.

The Battle of the Yarmuk is regarded as one of the most decisive battles in military history, and it marked the first great wave of early Muslim conquests after the death of the Islamic prophet Muhammad, heralding the rapid advance of Islam into the then-Christian Levant. The battle is widely regarded to be Khalid ibn al-Walid's greatest military victory and cemented his reputation as one of the greatest tacticians and cavalry commanders in history.

The Byzantine army, composed of the survivors of Yarmouk and other Syrian campaigns, was defeated, retreating to Antioch, whereupon the Muslims besieged the city. Having little hope of help from the Emperor, Antioch surrendered on 30 October, on the condition that all Byzantine troops would be given safe passage to Constantinople.

Emperor Heraclius had already left Antioch for Edessa before the Muslims arrived. He then arranged for the necessary defenses in Jazirah and left for Constantinople. On the way, he had a narrow escape when Khalid, who had just captured Marash, was heading south towards Manbij. Heraclius hastily took the mountainous path and, on passing through the Cilician gates, is reported to have said, "Farewell, a long farewell to Syria, my fair province. Thou art an infidel's (enemy's) now. Peace be with you, O, Syria – what a beautiful land you will be for the enemy hands."

The Muslim conquest of Egypt, also known as the Rashidun conquest of Egypt, led by the army of 'Amr ibn al-'As, took place between 639 and 646 and was overseen by the Rashidun Caliphate. It ended the seven centuries long period of Roman/Byzantine reign over Egypt that began in 30 BCE. Byzantine rule in the country had been shaken, as Egypt had been conquered and occupied for a decade by the Sassanid Iran in 618–629, before being recovered by the Byzantine emperor Heraclius. The caliphate took advantage of Byzantines' exhaustion and captured Egypt ten years after its reconquest by Heraclius. During the mid-630s, Byzantium had already lost the Levant and its Ghassanid allies in Arabia to the Caliphate. The loss of the prosperous province of Egypt and the defeat of the Byzantine armies severely weakened the empire, resulting in further territorial losses in the centuries to come.

The Battle of Heliopolis or Ayn Shams was a decisive battle between Arab Muslim armies and Byzantine forces for the control of Egypt. Though there were several major skirmishes after this battle, it effectively decided the fate of the Byzantine rule in Egypt, and opened the door for the Muslim conquest of the Byzantine Exarchate of Africa.

Constans II, nicknamed "the Bearded", was emperor of the Byzantine Empire from 641 to 668. He was the last attested emperor to serve as consul, in 642, although the office continued to exist until the reign of Leo VI the Wise (r. 886–912).

Under Constans, the Byzantines completely withdrew from Egypt in 642. Constans attempted to steer a middle line in the church dispute between Orthodoxy and Monothelitism by refusing to persecute either and prohibiting further discussion of the natures of Jesus Christ by decree in 648 (the Type of Constans). In 654, however, Mu'awiya renewed his raids by sea, plundering Rhodes. Constans led a fleet to attack the Muslims at Phoinike (off Lycia) in 655 at the Battle of the Masts, but he was defeated: 500 Byzantine ships were destroyed in the battle, and the Emperor himself was almost killed.;In 658, with the eastern frontier under less pressure, Constans defeated the Slavs in the Balkans, temporarily reasserting some notion of Byzantine rule over them and resettled some of them in Anatolia (ca. 649 or 667). In 659 he campaigned far to the east, taking advantage of a rebellion against the Caliphate in Media. The same year he concluded peace with the Arabs. However, having attracted the hatred of the citizens of Constantinople, Constans decided to leave the capital and to move to Syracuse in Sicily.On his way, he stopped in Greece and fought the Slavs at Thessalonica with success. Then, in the winter of 662–663, he made his camp at Athens. From there, in 663, he continued to Italy. In 663 Constans visited Rome for twelve days—the only emperor to set foot in Rome for two centuries—and was received with great honor by Pope Vitalian (657–672).;

Chinese histories for the Tang dynasty (618–907 CE) record contacts with merchants from "Fulin", the new name used to designate the Byzantine Empire. The first reported diplomatic contact took place in 643 CE during the reigns of Constans II (641–668 CE) and Emperor Taizong of Tang (626–649 CE). The Old Book of Tang, followed by the New Book of Tang, provides the name "Po-to-li" for Constans II, which Hirth conjectured to be a transliteration of Kōnstantinos Pogonatos, or "Constantine the Bearded", giving him the title of a king.

The Tang histories record that Constans II sent an embassy in the 17th year of the Zhenguan regnal period (643 CE), bearing gifts of red glass and green gemstones. Yule points out that Yazdegerd III (r. 632–651 CE), last ruler of the Sasanian Empire, sent diplomats to China to secure aid from Emperor Taizong (considered the suzerain over Ferghana in Central Asia) during the loss of the Persian heartland to the Islamic Rashidun Caliphate, which may also have prompted the Byzantines to send envoys to China amid their recent loss of Syria to the Muslims. Tang Chinese sources also recorded how Sasanian prince Peroz III (636–679 CE) fled to Tang China following the conquest of Persia by the growing Islamic caliphate.

Following their victory at the Battle of Heliopolis in July 640, and the subsequent capitulation of Alexandria in November 641, Arab troops had taken over what was the Roman province of Egypt. The newly installed Byzantine Emperor Constans II was determined to retake the land, and ordered a large fleet to carry troops to Alexandria. These troops, under Manuel, took the city by surprise from its small Arab garrison towards the end of 645 in an amphibious attack. In 645, the Byzantine thus temporarily won Alexandria back. Amr at the time might have been in Mecca, and was quickly recalled to take command of the Arab forces in Egypt.

The battle took place at the small fortified town of Nikiou, about two-thirds of the way from Alexandria to Fustat, with the Arab forces numbering around 15,000, against a smaller Byzantine force. The Arabs prevailed, and the Byzantine forces retreated in disarray, back to Alexandria. Although the Byzantines closed the gates against the pursuing Arabs, the city of Alexandria eventually fell to the Arabs, who stormed the city sometime in the summer of that year.

The permanent loss of the Egypt left the Byzantine Empire without an irreplaceable source of food and money. The new center for manpower and revenue shifts to Anatolia. The loss of Egypt and Syria, followed later by the conquest of the Exarchate of Africa also meant that the Mediterranean, long a "Roman lake", was now contested between two powers: the Muslim Caliphate and the Byzantines.

In 647, a Rashidun-Arab army led by Abdallah ibn al-Sa’ad invaded the Byzantine Exarchate of Africa. Tripolitania was conquered, followed by Sufetula, 150 miles (240 km) south of Carthage, and the governor and self-proclaimed Emperor of Africa Gregory was killed. Abdallah's booty-laden force returned to Egypt in 648 after Gregory's successor, Gennadius, promised them an annual tribute of some 300,000 nomismata.

The Typos of Constans (also called Type of Constans) was an edict issued by eastern Roman emperor Constans II in 648 in an attempt to defuse the confusion and arguments over the Christological doctrine of Monotheletism. For over two centuries, there had been a bitter debate regarding the nature of Christ: the orthodox Chalcedonian position defined Christ as having two natures in one person, whereas Miaphysite opponents contended that Jesus Christ possessed but a single nature. At the time, the Byzantine Empire had been at near constant war for fifty years and had lost large territories. It was under great pressure to establish domestic unity. This was hampered by the large number of Byzantines who rejected the Council of Chalcedon in favour of Monophysitism. The Typos attempted to dismiss the entire controversy, on pain of dire punishment. This extended to kidnapping the Pope from Rome to try him for high treason and mutilating one of the Typos's main opponents. Constans died in 668.

In 654, Muawiyah undertook an expedition in Cappadocia while his fleet, under the command of Abu'l-Awar, advanced along the southern coast of Anatolia. Emperor Constans embarked against it with a large fleet. Due to the rough seas, Tabari describes the Byzantine and Arab ships being arranged in lines and lashed together, to allow for melee combat. The Arabs were victorious in battle, although losses were heavy for both sides, and Constans barely escaped to Constantinople. According to Theophanes, he managed to make his escape by exchanging uniforms with one of his officers.

The battle was part of the earliest campaign by Muawiyah to reach Constantinople and is considered to be "the first decisive conflict of Islam on the deep". The Muslim victory was a significant event in the naval history of the Mediterranean Sea. From long being considered a ‘Roman lake’, the Mediterranean became a contending point between the naval might of the rising Rashidun Caliphate and the Eastern Roman Empire. The victory also paved the path for uncontested Muslim expansion along the coastline of North Africa.

During Umar's reign, the governor of Syria, Muawiyah I, sent a request to build a naval force to invade the islands of the Mediterranean Sea but Umar rejected the proposal because of the risk to the soldiers. Once Uthman became caliph, however, he approved Muawiyah's request. In 650, Muawiyah attacked Cyprus, conquering the capital, Constantia, after a brief siege, but signed a treaty with the local rulers. During this expedition, a relative of Muhammad, Umm-Haram, fell from her mule near the Salt Lake at Larnaca and was killed. She was buried in that same spot, which became a holy site for many local Muslims and Christians and, in 1816, the Hala Sultan Tekke was built there by the Ottomans. After apprehending a breach of the treaty, the Arabs re-invaded the island in 654 with five hundred ships. This time, however, a garrison of 12,000 men was left in Cyprus, bringing the island under Muslim influence. After leaving Cyprus, the Muslim fleet headed towards Crete and then Rhodes and conquered them without much resistance. From 652 to 654, the Muslims launched a naval campaign against Sicily and captured a large part of the island. Soon after this, Uthman was murdered, ending his expansionist policy, and the Muslims accordingly retreated from Sicily. In 655 Byzantine Emperor Constans II led a fleet in person to attack the Muslims at Phoinike (off Lycia) but it was defeated: both sides suffered heavy losses in the battle, and the emperor himself narrowly avoided death.

The First Fitna was the first civil war in the Islamic community which led to the overthrow of the Rashidun Caliphate and the establishment of the Umayyad Caliphate. The civil war involved three main battles between the fourth Rashidun caliph, Ali, and the rebel groups. The roots of the first civil war can be traced back to the assassination of the second caliph, Umar. Before he died from his wounds, Umar formed a six-member council, which ultimately elected Uthman as the next caliph. During the final years of Uthman's caliphate, he was accused of nepotism and eventually killed by rebels in 656. After Uthman's assassination, Ali was elected the fourth caliph. Aisha, Talha, and Zubayr revolted against Ali to depose him. The two parties fought the Battle of the Camel in December 656, in which Ali emerged victorious. Afterwards, Mu'awiya, the incumbent governor of Syria, declared war on Ali ostensibly to avenge Uthman's death. The two parties fought the Battle of Siffin in July 657.

Constans grew increasingly fearful that his younger brother, Theodosius, could oust him from the throne; he therefore obliged Theodosius to take holy orders and later had him killed in 660. However, having attracted the hatred of the citizens of Constantinople, Constans decided to leave the capital and to move to Syracuse in Sicily.

On his way, he stopped in Greece and fought the Slavs at Thessalonica with success. Then, in the winter of 662–663, he made his camp at Athens. From there, in 663, he continued to Italy. He launched an assault against the Lombard Duchy of Benevento, which then encompassed most of Southern Italy. Taking advantage of the fact that Lombard king Grimoald I of Benevento was engaged against Frankish forces from Neustria, Constans disembarked at Taranto and besieged Lucera and Benevento. However, the latter resisted and Constans withdrew to Naples. During the journey from Benevento to Naples, Constans II was defeated by Mitolas, Count of Capua, near Pugna. Constans ordered Saburrus, the commander of his army, to attack the Lombards again, but he was defeated by the Beneventani at Forino, between Avellino and Salerno. In 663 Constans visited Rome for twelve days—the only emperor to set foot in Rome for two centuries—and was received with great honor by Pope Vitalian (657–672).

As early as 668 the Caliph Muawiyah I received an invitation from Saborios, the commander of the troops in Armenia, to help overthrow the Emperor at Constantinople. He sent an army under his son Yazid against the Byzantine Empire. Yazid reached Chalcedon and took the important Byzantine center Amorion. While the city was quickly recovered, the Arabs next attacked Carthage and Sicily in 669. In 670 the Arabs captured Cyzicus and set up a base from which to launch further attacks into the heart of the Empire. Their fleet captured Smyrna and other coastal cities in 672.

On 15 July 668, Contans II was assassinated in his bath by his chamberlain, according to Theophilus of Edessa, with a bucket. His son Constantine succeeded him as Constantine IV. A brief usurpation in Sicily by Mezezius was quickly suppressed by the new emperor. Constantine IV was Byzantine Emperor from 668 to 685. His reign saw the first serious check to nearly 50 years of uninterrupted Islamic expansion, while his calling of the Sixth Ecumenical Council saw the end of the monothelitism controversy in the Byzantine Empire; for this, he is venerated as a saint in the Eastern Orthodox Church, with his feast day on September 3. He successfully defended Constantinople from the Arabs.

Under Mu'awiya's direction, the Muslim conquest of Ifriqiya (central North Africa) was launched by the commander Uqba ibn Nafi in 670, which extended Umayyad control as far as Byzacena (modern southern Tunisia), where Uqba founded the permanent Arab garrison city of Kairouan.

The first Arab siege of Constantinople in 674–678 was a major conflict of the Arab–Byzantine wars, and the first culmination of the Umayyad Caliphate's expansionist strategy towards the Byzantine Empire, led by Caliph Mu'awiya I. Mu'awiya, who had emerged in 661 as the ruler of the Muslim Arab empire following a civil war, renewed aggressive warfare against Byzantium after a lapse of some years and hoped to deliver a lethal blow by capturing the Byzantine capital, Constantinople.

As reported by the Byzantine chronicler Theophanes the Confessor, the Arab attack was methodical: in 672–673 Arab fleets secured bases along the coasts of Asia Minor, and then proceeded to install a loose blockade around Constantinople. They used the peninsula of Cyzicus near the city as a base to spend the winter, and returned every spring to launch attacks against the city's fortifications. Finally, the Byzantines, under Emperor Constantine IV, managed to destroy the Arab navy using a new invention, the liquid incendiary substance known as Greek fire. The Byzantines also defeated the Arab land army in Asia Minor, forcing them to lift the siege. The Byzantine victory was of major importance for the survival of the Byzantine state, as the Arab threat receded for a time. A peace treaty was signed soon after, and following the outbreak of another Muslim civil war, the Byzantines even experienced a period of ascendancy over the Caliphate.

The siege of Thessalonica (676–678 CE) occurred amidst the backdrop of increasing Slavic presence and pressure on the Byzantine Empire. Initial Slavic incursions began during the reign of Justinian I (527–565 CE), escalating with the support of the Avar Khaganate in the 560s, leading to significant settlements in the Balkans. The Byzantine Empire's focus on Eastern conflicts and internal strife facilitated Slavic and Avar advances, culminating in a notable presence around Thessalonica by the 610s, effectively isolating the city.

By the mid-7th century, cohesive Slavic entities, or Sclaviniae, had formed, challenging Byzantine control. The Byzantine response included military campaigns and relocation of Slavs to Asia Minor by Emperor Constans II in 658. Tensions with the Slavs intensified when Perboundos, a Slavic leader, was arrested and later executed by the Byzantines, sparking a revolt. This led to a coordinated siege by Slavic tribes on Thessalonica, exploiting the Byzantine preoccupation with Arab threats.

The siege, characterized by frequent raids and a blockade, strained the city through famine and isolation. Despite the dire situation, miraculous interventions attributed to Saint Demetrius and strategic military and diplomatic responses by the Byzantines, including a relief expedition, eventually alleviated the city's plight. The Slavs continued raids but shifted focus to naval engagements until the Byzantine military, finally able to address the Slavic threat post-Arab conflict, decisively countered the Slavs in Thrace.

Scholarly debate on the siege's precise chronology has varied, with current consensus favoring 676–678 CE, aligned with the First Arab Siege of Constantinople. This period marks a significant episode in the Byzantine-Slavic interactions, highlighting the complexities of medieval Balkan politics and the resilience of Thessalonica amidst external pressures.

Over the following five years, the Arabs returned each spring to continue the siege of Constantinople, but with the same results. The city survived, and finally in 678 the Arabs were forced to raise the siege. The Arabs withdrew and were almost simultaneously defeated on land in Lycia in Anatolia. This unexpected reverse forced Muawiyah I to seek a truce with Constantine. The terms of the concluded truce required the Arabs to evacuate the islands they had seized in the Aegean, and for the Byzantines to pay an annual tribute to the Caliphate consisting of fifty slaves, fifty horses, and 300,000 nomismata. The raising of the siege allowed Constantine to go to the relief of Thessalonica, still under siege from the Sclaveni.

The Third Council of Constantinople, counted as the Sixth Ecumenical Council by the Eastern Orthodox and Catholic Churches, as well by certain other Western Churches, met in 680–681 and condemned monoenergism and monothelitism as heretical and defined Jesus Christ as having two energies and two wills (divine and human).

In 680, the Bulgars under Khan Asparukh crossed the Danube into nominally Imperial territory and began to subjugate the local communities and Slavic tribes. In 680, Constantine IV led a combined land and sea operation against the invaders and besieged their fortified camp in Dobruja. Suffering from bad health, the Emperor had to leave the army, which panicked and;was defeated;at the hands of Asparuh at Onglos, a swampy region in or around the Danube Delta where the Bulgars had set a fortified camp. The Bulgars advanced south, crossed the Balkan Mountains and invaded Thrace.

In 681, the Byzantines were compelled to sign a humiliating peace treaty, forcing them to acknowledge Bulgaria as an independent state, to cede the territories to the north of the Balkan Mountains and to pay an annual tribute. In his universal chronicle the Western European author Sigebert of Gembloux remarked that the Bulgarian state was established in 680. This was the first state that the empire recognised in the Balkans and the first time it legally surrendered claims to part of its Balkan dominions.

Justinian II was the last Byzantine emperor of the Heraclian dynasty, reigning from 685 to 695 and again from 705 to 711. Like Justinian I, Justinian II was an ambitious and passionate ruler who was keen to restore the Roman Empire to its former glories, but he responded brutally to any opposition to his will and lacked the finesse of his father, Constantine IV. Consequently, he generated enormous opposition to his reign, resulting in his deposition in 695 in a popular uprising. He only returned to the throne in 705 with the help of a Bulgar and Slav army. His second reign was even more despotic than the first, and it too saw his eventual overthrow in 711. He was abandoned by his army, who turned on him before killing him.

The civil war in the Umayyad Caliphate provided an opportunity for the Byzantine Empire to attack its weakened rival, and, in 686, Emperor Justinian II sent Leontios to invade Umayyad territory in Armenia and Iberia, where he campaigned successfully, before leading troops in Adharbayjan and Caucasian Albania; during these campaigns he gathered loot. Leontios' successful campaigns compelled the Umayyad Caliph, Abd al-Malik ibn Marwan, to sue for peace in 688, agreeing to tender part of the taxes from Umayyad territory in Armenia, Iberia, and Cyprus, and to renew a treaty signed originally under Constantine IV, providing for a weekly tribute of 1,000 pieces of gold, one horse, and one slave.

Due to Constantine IV's victories, the situation in the Eastern provinces of the Empire was stable when Justinian ascended the throne. After a preliminary strike against the Arabs in Armenia, Justinian managed to augment the sum paid by the Umayyad Caliphs as an annual tribute, and to regain control of part of Cyprus. The incomes of the provinces of Armenia and Iberia were divided among the two empires. Justinian signed a treaty with the Caliph Abd al-Malik ibn Marwan which rendered Cyprus neutral ground, with its tax revenue split.

Justinian took advantage of the peace in the East to regain possession of the Balkans, which were before then almost totally under the heel of Slavic tribes. In 687 Justinian transferred cavalry troops from Anatolia to Thrace. With a great military campaign in 688–689, Justinian defeated the Bulgars of Macedonia and was finally able to enter Thessalonica, the second most important Byzantine city in Europe.

After subduing Slavs, many were resettled in Anatolia, where they were to provide a military force of 30,000 men. Emboldened by the increase of his forces in Anatolia, Justinian now renewed the war against the Arabs. With the help of his new troops, Justinian won a battle against the enemy in Armenia in 693, but they were soon bribed to revolt by the Arabs.

The Umayyad army was led by Muhammad ibn Marwan. The Byzantines were led by Leontios and included a "special army" of 30,000 Slavs under their leader, Neboulos. The Umayyads, incensed at the breaking of the treaty, used copies of its texts in the place of a flag. Though the battle seemed to be tilting to the Byzantine advantage, the defection of upwards of 20,000 Slavs ensured a Byzantine defeat. Justinian was forced to flee to the Propontis. As a result, Justinian imprisoned Leontios for this defeat.

While Justinian II's land policies threatened the aristocracy, his tax policy was very unpopular with the common people. Through his agents Stephen and Theodotos, the emperor raised the funds to gratify his sumptuous tastes and his mania for erecting costly buildings. This, ongoing religious discontent, conflicts with the aristocracy, and displeasure over his resettlement policy eventually drove his subjects into rebellion. In 695 the population rose under Leontios, the strategos of Hellas, and proclaimed him Emperor. Justinian was deposed and his nose was cut off (later replaced by a solid gold replica of his original) to prevent his again seeking the throne: such mutilation was common in Byzantine culture. He was exiled to Cherson in the Crimea.

The Umayyads, emboldened by Leontius' perceived weakness, invaded the Exarchate of Africa in 696, capturing Carthage in 697. Leontius sent the patrikios John to retake the city. John was able to seize Carthage after a surprise attack on its harbor. However, Umayyad reinforcements soon retook the city, forcing John to retreat to Crete and regroup. A group of officers, fearing the Emperor's punishment for their failure, revolted and proclaimed Apsimar, a droungarios (mid-level commander) of the Cibyrrhaeots, emperor.

Apsimar took the regnal name Tiberius, gathered a fleet and allied himself with the Green faction, before sailing for Constantinople, which was enduring the bubonic plague. After several months of siege, the city surrendered to Tiberius, in 698. The Chronicon Altinate gives the date February 15. Tiberius captured Leontius, and had his nose slit before imprisoning him in the Monastery of Dalmatou.

Tiberius III was Byzantine emperor from 15 February 698 to 10 July or 21 August 705 CE. In 696, Tiberius was part of an army led by John the Patrician sent by Byzantine Emperor Leontios to retake the city of Carthage in the Exarchate of Africa, which had been captured by the Arab Umayyads. After seizing the city, this army was pushed back by Umayyad reinforcements and retreated to the island of Crete; some of the officers, fearing the wrath of Leontios, killed John and declared Tiberius emperor. Tiberius swiftly gathered a fleet, sailed for Constantinople, and deposed Leontios. Tiberius did not attempt to retake Byzantine Africa from the Umayyads, but campaigned against them along the eastern border with some success.

The Armenians launched a large revolt against the Umayyads in 702, requesting Byzantine aid. Abdallah ibn Abd al-Malik launched a campaign to reconquer Armenia in 704 but was attacked by Heraclius, brother of Emperor Tiberius III in Cilicia. Heraclius defeated the Arab army of 10,000–12,000 men led by Yazid ibn Hunain at Sisium, killing most and enslaving the rest; however, Heraclius was not able to stop Abdallah ibn Abd al-Malik from reconquering Armenia.

Justinian II approached Tervel of Bulgaria who agreed to provide all the military assistance necessary for Justinian to regain his throne in exchange for financial considerations, the award of a Caesar's crown, and the hand of Justinian's daughter, Anastasia, in marriage. In spring 705, with an army of 15,000 Bulgar and Slav horsemen, Justinian appeared before the walls of Constantinople. For three days, Justinian tried to convince the citizens of Constantinople to open the gates, but to no avail. Unable to take the city by force, he and some companions entered through an unused water conduit under the walls of the city, roused their supporters, and seized control of the city in a midnight coup d'état. Justinian once more ascended the throne, breaking the tradition preventing the mutilated from Imperial rule. After tracking down his predecessors, he had his rivals Leontius and Tiberius brought before him in chains in the Hippodrome. There, before a jeering populace, Justinian, now wearing a golden nasal prosthesis, placed his feet on the necks of Tiberius and Leontius in a symbolic gesture of subjugation before ordering their execution by beheading, followed by many of their partisans, as well as deposing, blinding and exiling Patriarch Kallinikos I of Constantinople to Rome.

In 708 Justinian turned on Bulgarian Khan Tervel, whom he had earlier crowned Caesar, and invaded Bulgaria, apparently seeking to recover the territories ceded to Tervel as a reward for his support in 705. The Emperor was defeated, blockaded in Anchialus, and forced to retreat. Peace between Bulgaria and Byzantium was quickly restored.;

The cities of Cilicia fell into the hands of the Umayyads, who penetrated into Cappadocia in 709–711. The region had, however, been almost completely depopulated already since the middle of the 7th century and formed a no man's land between the Romans and the Caliphate. The western parts of the old province of Cilicia remained in Roman hands and became part of the Cibyrrhaeot Theme. The status quo would remain unchanged for over 260 years before Cilicia was eventually reconquered for the Romans in the 950s and 960s by Nikephoros Phokas and John Tzimiskes.

Justinian II's rule provoked another uprising against him. Cherson revolted, and under the leadership of the exiled general Bardanes the city held out against a counter-attack. Soon, the forces sent to suppress the rebellion joined it. The rebels then seized the capital and proclaimed Bardanes as Emperor Philippicus; Justinian had been on his way to Armenia, and was unable to return to Constantinople in time to defend it. He was arrested and executed in November 711, his head being exhibited in Rome and Ravenna.

Justinian's reign saw the continued slow and ongoing process of transformation of the Byzantine Empire, as the traditions inherited from the ancient Latin Roman state were gradually being eroded. A pious ruler, Justinian was the first emperor to include the image of Christ on coinage issued in his name and attempted to outlaw various pagan festivals and practices that persisted in the Empire. He may have self-consciously modelled himself on his namesake, Justinian I, as seen in his enthusiasm for large-scale construction projects and the renaming of his Khazar wife with the name of Theodora.