The Roman army succeeded in conquering many territories covering the Mediterranean region and coastal regions in southwestern Europe and North Africa. These territories were home to many different cultural groups, both urban populations, and rural populations. The West also suffered more heavily from the instability of the 3rd century. This distinction between the established Hellenised East and the younger Latinised West persisted and became increasingly important in later centuries, leading to a gradual estrangement of the two worlds.

An early instance of the partition of the Empire into East and West occurred in 293 when Emperor Diocletian created a new administrative system (the tetrarchy), to guarantee security in all endangered regions of his Empire. He associated himself with a co-emperor (Augustus), and each co-emperor then adopted a young colleague given the title of Caesar, to share in their rule and eventually to succeed the senior partner. Each tetrarch was in charge of a part of the Empire.

The tetrarchy collapsed, however, in 313 and a few years later Constantine I reunited the two administrative divisions of the Empire as sole Augustus.

Constantine established the principle that emperors could not settle questions of doctrine on their own but should instead summon general ecclesiastical councils for that purpose. His convening of both the Synod of Arles and the First Council of Nicaea indicated his interest in the unity of the Church and showcased his claim to be its head.

In 330, Constantine moved the seat of the Empire to Constantinople, which he founded as a second Rome on the site of Byzantium, a city strategically located on the trade routes between Europe and Asia and between the Mediterranean and the Black Sea.

The rise of Christianity was briefly interrupted by the accession of the emperor Julian in 361, who made a determined effort to restore polytheism throughout the empire and was thus dubbed "Julian the Apostate" by the Church.

However, this was reversed when Julian was killed in battle in 363.

Theodosius I (379–395) was the last Emperor to rule both the Eastern and Western halves of the Empire.

In 391 and 392, Theodosius issued a series of edicts essentially banning pagan religion. Pagan festivals and sacrifices were banned, as was access to all pagan temples and places of worship.

The last Olympic Games are believed to have been held in 393.

In 395, Theodosius I bequeathed the imperial office jointly to his sons: Arcadius in the East and Honorius in the West, once again dividing Imperial administration.

This success allowed Theodosius II to focus on codifying Roman law with the Codex Theodosianus.

Theodosius II focused on fortification of the walls of Constantinople, which left the city impervious to most attacks until 1204. Large portions of the Theodosian Walls are preserved to the present day.

To fend off the Huns, Theodosius had to pay an enormous annual tribute to Attila. His successor, Marcian, refused to continue to pay the tribute, but Attila had already diverted his attention to the Western Roman Empire. After Attila's death in 453, the Hun Empire collapsed, and many of the remaining Huns were often hired as mercenaries by Constantinople.

After the fall of Attila, the Eastern Empire enjoyed a period of peace, while the Western Empire continued to deteriorate due to the expanding migration and invasions of the barbarians, most prominently the Germanic nations. The West's end is usually dated 476 when the East Germanic Roman foederati general Odoacer deposed the Western Emperor Romulus Augustulus, a year after the latter usurped the position from Julius Nepos.

In 480 with the death of Julius Nepos, Eastern Emperor Zeno became the sole claimant to Emperor of the empire. Odoacer, now the ruler of Italy, was nominally Zeno's subordinate but acted with complete autonomy, eventually providing support to a rebellion against the Emperor.

In 491, Anastasius I, an aged civil officer of Roman origin, became Emperor, but it was not until 497 that the forces of the new emperor effectively took the measure of Isaurian resistance. Anastasius revealed himself as an energetic reformer and an able administrator. He introduced a new coinage system of the copper follis, the coin used in most everyday transactions. He also reformed the tax system and permanently abolished the chrysargyron tax.

Zeno negotiated with the invading Ostrogoths, who had settled in Moesia, convincing the Gothic king Theodoric to depart for Italy as magister militum per Italiam ("commander in chief for Italy") to depose Odoacer. After Odoacer's defeat in 493, Theodoric ruled Italy de facto, although he was never recognized by the eastern emperors as "king" (rex).

The State Treasury contained the enormous sum of 320,000 lb (150,000 kg) of gold when Anastasius died in 518.

The Justinian dynasty was founded by Justin I, who though illiterate, rose through the ranks of the military to become Emperor in 518.

Justin I was succeeded by his nephew Justinian I in 527, who may already have exerted effective control during Justin's reign. One of the most important figures of late antiquity and possibly the last Roman emperor to speak Latin as a first language, Justinian's rule constitutes a distinct epoch, marked by the ambitious but only partly realized renovatio imperii, or "restoration of the Empire". His wife Theodora was particularly influential.

In 529, Justinian appointed a ten-man commission chaired by John the Cappadocian to revise Roman law and create a new codification of laws and jurists' extracts, known as the "Corpus Juris Civilis", or the Justinian Code.

Although polytheism had been suppressed by the state since at least the time of Constantine in the 4th century, traditional Greco-Roman culture was still influential in the Eastern empire in the 6th century. Hellenistic philosophy began to be gradually amalgamated into newer Christian philosophy. Philosophers such as John Philoponus drew on Neoplatonic ideas in addition to Christian thought and empiricism. Because of the active paganism of its professors, Justinian closed down the Neoplatonic Academy in 529. Other schools continued in Constantinople, Antioch, and Alexandria, which were the centers of Justinian's empire.

In 532, attempting to secure his eastern frontier, Justinian signed a peace treaty with Khosrau I of Persia, agreeing to pay a large annual tribute to the Sassanids.

Justinian survived a revolt in Constantinople (the Nika riots), which solidified his power but ended with the deaths of a reported 30,000 to 35,000 rioters on his orders.

The western conquests began in 533, as Justinian sent his general Belisarius to reclaim the former province of Africa from the Vandals, who had been in control since 429 with their capital at Carthage Their success came with surprising ease, but it was not until 548 that the major local tribes were subdued.

In 534, the Corpus was updated and, along with the enactments promulgated by Justinian after 534, formed the system of law used for most of the rest of the Byzantine era. The Corpus forms the basis of civil law of many modern states. The Corpus forms the basis of civil law of many modern states.

In 535, a small Byzantine expedition to Sicily met with easy success, but the Goths soon stiffened their resistance.

In 535–536, Theodahad sent Pope Agapetus I to Constantinople to request the removal of Byzantine forces from Sicily, Dalmatia, and Italy. Although Agapetus failed in his mission to sign a peace with Justinian, he succeeded in having the Monophysite Patriarch Anthimus I of Constantinople denounced, despite Empress Theodora's support and protection.

Hagia Sophia built in 537, during the reign of Justinian.

Victory did not come until 538, when Belisarius captured Ravenna, after successful sieges of Naples and Rome.

The Ostrogoths captured Rome in 546.

By the mid-550s, Justinian had won victories in most theatres of operation, with the notable exception of the Balkans, which were subjected to repeated incursions from the Slavs and the Gepids. Tribes of Serbs and Croats were later resettled in the northwestern Balkans, during the reign of Heraclius. Justinian called Belisarius out of retirement and defeated the new Hunnish threat. The strengthening of the Danube fleet caused the Kutrigur Huns to withdraw and they agreed to a treaty that allowed safe passage back across the Danube.

In 551, Athanagild, a noble from Visigothic Hispania, sought Justinian's help in a rebellion against the king, and the emperor dispatched a force under Liberius, a successful military commander. The empire held on to a small slice of the Iberian Peninsula coast until the reign of Heraclius.

The arrival of the Armenian eunuch Narses in Italy (late 551) with an army of 35,000 men marked another shift in Gothic fortunes. Totila of the Ostrogoths was defeated at the Battle of Taginae.

Totila's successor, Teia, was defeated at the Battle of Mons Lactarius (October 552).

In the east, the Roman–Persian Wars continued until 562 when the envoys of Justinian and Khosrau agreed on a 50-year peace.

After Justinian died in 565, his successor, Justin II, refused to pay the large tribute to the Persians. Meanwhile, the Germanic Lombards invaded Italy; by the end of the century, only a third of Italy was in Byzantine hands.

Justin's successor, Tiberius II, choosing between his enemies, awarded subsidies to the Avars while taking military action against the Persians. Although Tiberius' general, Maurice, led an effective campaign on the eastern frontier, subsidies failed to restrain the Avars. They captured the Balkan fortress of Sirmium in 582, while the Slavs began to make inroads across the Danube.

Maurice, who meanwhile succeeded Tiberius, intervened in a Persian civil war, placed the legitimate Khosrau II back on the throne, and married his daughter to him. Maurice's treaty with his new brother-in-law enlarged the territories of the Empire to the East and allowed the energetic Emperor to focus on the Balkans.

By 602, a series of successful Byzantine campaigns had pushed the Avars and Slavs back across the Danube.

Maurice's refusal to ransom several thousand captives taken by the Avars, and his order to the troops to winter in the Danube, caused his popularity to plummet. A revolt broke out under an officer named Phocas, who marched the troops back to Constantinople; Maurice and his family were murdered while trying to escape.

After Maurice's murder by Phocas, Khosrau used the pretext to reconquer the Roman province of Mesopotamia. Phocas, an unpopular ruler invariably described in Byzantine sources as a "tyrant", was the target of a number of Senate-led plots. He was eventually deposed in 610 by Heraclius, who sailed to Constantinople from Carthage with an icon affixed to the prow of his ship.

Following the accession of Heraclius, the Sassanid advance pushed deep into the Levant, occupying Damascus and Jerusalem and removing the True Cross to Ctesiphon.

The city also lost the free grain shipments in 618, after Egypt fell first to the Persians and then to the Arabs, and public wheat distribution ceased.

The counter-attack launched by Heraclius took on the character of a holy war, and an acheiropoietos image of Christ was carried as a military standard (similarly, when Constantinople was saved from a combined Avar–Sassanid–Slavic siege in 626, the victory was attributed to the icons of the Virgin that were led in procession by Patriarch Sergius about the walls of the city). In this very siege of Constantinople of the year 626, amidst the climactic Byzantine–Sasanian War of 602–628, the combined Avar, Sassanid, and Slavic forces unsuccessfully besieged the Byzantine capital between June and July. After this, the Sassanid army was forced to withdraw to Anatolia. The loss came just after news had reached them of yet another Byzantine victory, where Heraclius's brother Theodore scored well against the Persian general Shahin. Following this, Heraclius led an invasion into Sassanid Mesopotamia once again.

The main Sassanid force was destroyed at Nineveh in 627.

In 629, Heraclius restored the True Cross to Jerusalem in a majestic ceremony.

The Byzantines suffered a crushing defeat by the Arabs at the Battle of Yarmouk in 636.

The Muslim conquest of Egypt by the Arabs took place between 639 and 646 AD and was overseen by the Rashidun Caliphate. It ended the centuries-long period of Roman/Byzantine reign (beginning in 30 BC) over Egypt.

In the 670s, the Bulgars were pushed south of the Danube by the arrival of the Khazars.

The Arabs, now firmly in control of Syria and the Levant, sent frequent raiding parties deep into Asia Minor, and in 674–678 laid siege to Constantinople itself. The Arab fleet was finally repulsed through the use of Greek fire, and a thirty-years' truce was signed between the Empire and the Umayyad Caliphate. However, the Anatolian raids continued unabated, and accelerated the demise of classical urban culture, with the inhabitants of many cities either refortifying much smaller areas within the old city walls or relocating entirely to nearby fortresses.

In 680, Byzantine forces sent to disperse these new settlements were defeated.

In 681, Constantine IV signed a treaty with the Bulgar khan Asparukh, and the new Bulgarian state assumed sovereignty over several Slavic tribes that had previously, at least in name, recognized Byzantine rule.

In 687–688, the final Heraclian emperor, Justinian II, led an expedition against the Slavs and Bulgarians, and made significant gains, although the fact that he had to fight his way from Thrace to Macedonia demonstrates the degree to which Byzantine power in the north Balkans had declined.

Justinian II attempted to break the power of the urban aristocracy through severe taxation and the appointment of "outsiders" to administrative posts. He was driven from power in 695, and took shelter first with the Khazars and then with the Bulgarians.

In 705, Justinian II returned to Constantinople with the armies of the Bulgarian khan Tervel, retook the throne, and instituted a reign of terror against his enemies.

With his final overthrow in 711, supported once more by the urban aristocracy, the Heraclian dynasty came to an end.

In 717 the Umayyad Caliphate launched the siege of Constantinople (717–718) which lasted for one year. However, the combination of Leo III the Isaurian's military genius, the Byzantines' use of Greek Fire, cold winter in 717–718, and Byzantine diplomacy with the Khan Tervel of Bulgaria resulted in a Byzantine victory.

The 8th and early 9th centuries were also dominated by controversy and religious division over Iconoclasm, which was the main political issue in the Empire for over a century. Icons (here meaning all forms of religious imagery) were banned by Leo and Constantine V from around 730, leading to revolts by iconodules (supporters of icons) throughout the empire.

In 740 a major Byzantine victory took place at the Battle of Akroinon where the Byzantines destroyed the Umayyad army once again.

Leo III the Isaurian's son and successor, Constantine V, won noteworthy victories in northern Syria and also thoroughly undermined Bulgarian strength. In 746, profiting from the unstable conditions in the Umayyad Caliphate, which was falling apart under Marwan II, Constantine V invaded Syria and captured Germanikeia, and the Battle of Keramaia resulted in a major Byzantine naval victory over the Umayyad fleet. Coupled with military defeats on other fronts of the Caliphate and internal instability, Umayyad expansion came to an end.

After the efforts of empress Irene, the Second Council of Nicaea met in 787 and affirmed that icons could be venerated but not worshipped.

Under the leadership of emperor Krum, the Bulgarian threat also re-emerged, but in 815–816 Krum's son, Omurtag, signed a peace treaty with Leo V.

In the 830s Abbasid Caliphate started military excursions culminating with a victory in the Sack of Amorium.

In the early 9th century, Leo V reintroduced the policy of iconoclasm, but in 843 Empress Theodora restored the veneration of icons with the help of Patriarch Methodios. Iconoclasm played a part in the further alienation of East from West, which worsened during the so-called Photian schism when Pope Nicholas I challenged the elevation of Photios to the patriarchate.

The Byzantines then counter-attacked and sacked Damietta in Egypt.

The Rus' launched their first attack against Constantinople in 860, pillaging the suburbs of the city.

Taking advantage of the Empire's weakness after the Revolt of Thomas the Slav in the early 820s, the Arabs re-emerged and captured Crete. They also successfully attacked Sicily but in 863 general Petronas gained a decisive victory at the Battle of Lalakaon against Umar al-Aqta, the emir of Melitene (Malatya).

The accession of Basil I to the throne in 867 marks the beginning of the Macedonian dynasty, which ruled for 150 years.

In the early years of Basil I's reign, Arab raids on the coasts of Dalmatia and the siege of Ragusa (866–868) were defeated and the region once again came under secure Byzantine control.

By contrast, the Byzantine position in Southern Italy was gradually consolidated; by 873 Bari was once again under Byzantine rule and most of Southern Italy remained in the Empire for the next 200 years.

On the more important eastern front, the Empire rebuilt its defenses and went on the offensive. The Paulicians were defeated at the Battle of Bathys Ryax and their capital of Tephrike (Divrigi) taken.

While the offensive against the Abbasid Caliphate began with the recapture of Samosata.

Under Basil's son and successor, Leo VI the Wise, the wars in the east against the enfeebled Abbasid Caliphate continued.

Ending eighty years of peace between the two states, the powerful Bulgarian tsar Simeon I invaded in 894 but was pushed back by the Byzantines, who used their fleet to sail up the Black Sea to attack the Bulgarian rear, enlisting the support of the Hungarians.

The Byzantines were defeated at the Battle of Boulgarophygon in 896, however, and agreed to pay annual subsidies to the Bulgarians.

Sicily was lost to the Arabs in 902.

In 904 Thessaloniki, the Empire's second city was sacked by an Arab fleet.

The naval weakness of the Empire was rectified. Despite this revenge, the Byzantines were still unable to strike a decisive blow against the Muslims, who inflicted a crushing defeat on the imperial forces when they attempted to regain Crete in 911.

Leo the Wise died in 912, and hostilities soon resumed as Simeon marched to Constantinople at the head of a large army. Although the walls of the city were impregnable, the Byzantine administration was in disarray and Simeon was invited into the city, where he was granted the crown of basileus (emperor) of Bulgaria and had the young emperor Constantine VII marry one of his daughters. When a revolt in Constantinople halted his dynastic project, he again invaded Thrace and conquered Adrianople.

A great imperial expedition under Leo Phocas and Romanos I Lekapenos ended with another crushing Byzantine defeat at the Battle of Achelous in 917, and the following year the Bulgarians were free to ravage northern Greece.

Adrianople was plundered again in 923, and a Bulgarian army laid siege to Constantinople in 924.

The death of the Bulgarian tsar Simeon I in 927 severely weakened the Bulgarians, allowing the Byzantines to concentrate on the eastern front.

Melitene was permanently recaptured in 934.

In 941, they appeared on the Asian shore of the Bosphorus, but this time they were crushed, an indication of the improvements in the Byzantine military position after 907, when only diplomacy had been able to push back the invaders. The outcome was the Rus'–Byzantine Treaty of 945.

In 943 the famous general John Kourkouas continued the offensive in Mesopotamia with some noteworthy victories, culminating in the reconquest of Edessa. Kourkouas was especially celebrated for returning to Constantinople the venerated Mandylion, a relic purportedly imprinted with a portrait of Jesus.

The recapture of Crete in the siege of Chandax put an end to Arab raids in the Aegean, allowing mainland Greece to flourish again.

Nikephoros II Phokas took the great city of Aleppo in 962.

In 968, Bulgaria was overrun by the Rus' under Sviatoslav I of Kiev, but three years later, John I Tzimiskes defeated the Rus' and re-incorporated Eastern Bulgaria into the Byzantine Empire.

John I Tzimiskes recaptured Damascus, Beirut, Acre, Sidon, Caesarea, and Tiberias, putting Byzantine armies within striking distance of Jerusalem, although the Muslim power centers in Iraq and Egypt were left untouched.

Bulgarian resistance revived under the rule of the Cometopuli dynasty, but the new Emperor Basil II (r. 976–1025) made the submission of the Bulgarians his primary goal. Basil's first expedition against Bulgaria, however, resulted in a defeat at the Gates of Trajan. For the next few years, the emperor was preoccupied with internal revolts in Anatolia, while the Bulgarians expanded their realm in the Balkans.

Rus'–Byzantine relations became closer following the marriage of Anna Porphyrogeneta to Vladimir the Great in 988, and the subsequent Christianisation of the Rus'.

At the Battle of Kleidion in 1014 the Bulgarians were annihilated: their army was captured, and it is said that 99 out of every 100 men were blinded, with the hundredth man left with one eye so he could lead his compatriots home. When Tsar Samuil saw the broken remains of his once formidable army, he died of shock.

When Tsar Samuil saw the broken remains of his once formidable army, he died of shock. By 1018, the last Bulgarian strongholds had surrendered, and the country became part of the Empire.

After much campaigning in the north, the last Arab threat to Byzantium, the rich province of Sicily, was targeted in 1025 by Basil II, who died before the expedition could be completed. By that time the Empire stretched from the straits of Messina to the Euphrates and from the Danube to Syria.

Even after the Christianisation of the Rus', however, relations were not always friendly. The most serious conflict between the two powers was the war of 968–971 in Bulgaria, but several Rus' raiding expeditions against the Byzantine cities of the Black Sea coast and Constantinople itself are also recorded. Although most were repulsed, they were often followed by treaties that were generally favorable to the Rus', such as the one concluded at the end of the war of 1043, during which the Rus' indicated their ambitions to compete with the Byzantines as an independent power.

Between 1021 and 1022, following years of tensions, Basil II led a series of victorious campaigns against the Kingdom of Georgia, resulting in the annexation of several Georgian provinces to the Empire. Basil's successors also annexed Bagratid Armenia in 1045.

About 1053, Constantine IX disbanded what the historian John Skylitzes calls the "Iberian Army", which consisted of 50,000 men, and it was turned into a contemporary Drungary of the Watch. Two other knowledgeable contemporaries, the former officials Michael Attaleiates and Kekaumenos, agree with Skylitzes that by demobilizing these soldiers Constantine did catastrophic harm to the Empire's eastern defenses.

In 1054, relations between the Eastern and Western traditions of the Chalcedonian Christian Church reached a terminal crisis, known as the East-West Schism. Although there was a formal declaration of institutional separation, on 16 July, when three papal legates entered the Hagia Sophia during Divine Liturgy on a Saturday afternoon and placed a bull of excommunication on the altar, the so-called Great Schism was actually the culmination of centuries of gradual separation.

At the same time, Byzantium was faced with new enemies. Its provinces in southern Italy were threatened by the Normans, who arrived in Italy at the beginning of the 11th century. During a period of strife between Constantinople and Rome culminating in the East-West Schism of 1054, the Normans began to advance, slowly but steadily, into Byzantine Italy. Reggio, the capital of the tagma of Calabria, was captured in 1060 by Robert Guiscard, followed by Otranto in 1068. Bari, the main Byzantine stronghold in Apulia, was besieged in August 1068 and fell in April 1071.

The emergency lent weight to the military aristocracy in Anatolia, who in 1068 secured the election of one of their own, Romanos Diogenes, as emperor.

Importantly, both Georgia and Armenia were significantly weakened by the Byzantine administration's policy of heavy taxation and abolishing the levy. The weakening of Georgia and Armenia played a significant role in the Byzantine defeat from Seljuks at Manzikert. In the summer of 1071, Romanos undertook a massive eastern campaign to draw the Seljuks into a general engagement with the Byzantine army. At the Battle of Manzikert, Romanos suffered a surprise defeat by Sultan Alp Arslan, and was captured.

By 1081, the Seljuks had expanded their rule over virtually the entire Anatolian plateau from Armenia in the east to Bithynia in the west, and they had founded their capital at Nicaea, just 90 kilometers (56 miles) from Constantinople.

After Manzikert, a partial recovery (referred to as the Komnenian restoration) was made possible by the Komnenian dynasty. The Komnenoi attained power again under Alexios I in 1081. From the outset of his reign, Alexios faced a formidable attack by the Normans under Robert Guiscard and his son Bohemund of Taranto, who captured Dyrrhachium and Corfu, and laid siege to Larissa in Thessaly. Robert Guiscard's death in 1085 temporarily eased the Norman problem.

By his own efforts, Alexios defeated the Pechenegs, who were caught by surprise and annihilated at the Battle of Levounion on 28 April 1091.

At the Council of Piacenza in 1095, envoys from Alexios spoke to Pope Urban II about the suffering of the Christians of the East and underscored that without help from the West they would continue to suffer under Muslim rule. Urban saw Alexios's request as a dual opportunity to cement Western Europe and reunite the Eastern Orthodox Church with the Roman Catholic Church under his rule.

On 27 November 1095, Pope Urban II called together the Council of Clermont, and urged all those present to take up arms under the sign of the Cross and launch an armed pilgrimage to recover Jerusalem and the East from the Muslims. The response in Western Europe was overwhelming.

Alexios was able to recover a number of important cities, islands, and much of western Asia Minor. The Crusaders agreed to become Alexios' vassals under the Treaty of Devol in 1108, which marked the end of the Norman threat during Alexios' reign.

Alexios's son John II Komnenos succeeded him in 1118 and ruled until 1143. John was a pious and dedicated Emperor who was determined to undo the damage to the empire suffered at the Battle of Manzikert, half a century earlier.

During his twenty-five-year reign, John made alliances with the Holy Roman Empire in the West and decisively defeated the Pechenegs at the Battle of Beroia.

In 1142, John returned to press his claims to Antioch, but he died in the spring of 1143 following a hunting accident.

John's chosen heir was his fourth son, Manuel I Komnenos, who campaigned aggressively against his neighbors both in the west and in the east.

In an effort to restore Byzantine control over the ports of southern Italy, he sent an expedition to Italy in 1155, but disputes within the coalition led to the eventual failure of the campaign.

Despite this military setback, Manuel's armies successfully invaded the Southern parts of the Kingdom of Hungary in 1167, defeating the Hungarians at the Battle of Sirmium.

In the east, however, Manuel suffered a major defeat in 1176 at the Battle of Myriokephalon, against the Turks. Yet the losses were quickly recovered, and in the following year Manuel's forces inflicted a defeat upon a force of "picked Turks".

The Byzantine commander John Vatatzes, who destroyed the Turkish invaders at the Battle of Hyelion and Leimocheir, not only brought troops from the capital but also was able to gather an army along the way, a sign that the Byzantine army remained strong and that the defensive program of western Asia Minor was still successful.

Manuel's death on 24 September 1180 left his 11-year-old son Alexios II Komnenos on the throne. Alexios was highly incompetent in the office, and his mother, Maria of Antioch's Frankish background, made his regency unpopular.

Eventually, Andronikos I Komnenos, a grandson of Alexios I, launched a revolt against his younger relative and managed to overthrow him in a violent coup d'état. Utilizing his good looks and his immense popularity with the army, he marched on to Constantinople in 1182 and incited a massacre of the Latins.

After eliminating his potential rivals, Andronikos I Komnenos had himself crowned as co-emperor in September 1183. He eliminated Alexios II and took his 12-year-old wife Agnes of France for himself.

Despite his military background, Andronikos failed to deal with Isaac Komnenos, Béla III of Hungary (r. 1172–1196) who reincorporated Croatian territories into Hungary, and Stephen Nemanja of Serbia (r. 1166–1196) who declared his independence from the Byzantine Empire. Yet, none of these troubles would compare to William II of Sicily's (r. 1166–1189) invasion force of 300 ships and 80,000 men, arriving in 1185. Andronikos mobilized a small fleet of 100 ships to defend the capital, but other than that he was indifferent to the populace.

Andronikos I Komnenos was finally overthrown when Isaac Angelos, surviving an imperial assassination attempt, seized power with the aid of the people and had Andronikos killed.

The reign of Isaac II, and more so that of his brother Alexios III, saw the collapse of what remained of the centralized machinery of Byzantine government and defense. Although the Normans were driven out of Greece, in 1185 the Vlachs and Bulgars began a rebellion that led to the formation of the Second Bulgarian Empire. The internal policy of the Angeloi was characterized by the squandering of the public treasure and fiscal maladministration. Imperial authority was severely weakened, and the growing power vacuum at the center of the Empire encouraged fragmentation.

In 1198, Pope Innocent III broached the subject of a new crusade through legates and encyclical letters. The stated intent of the crusade was to conquer Egypt, now the center of Muslim power in the Levant.

The crusader army arrived at Venice in the summer of 1202 and hired the Venetian fleet to transport them to Egypt. As a payment to the Venetians, they captured the (Christian) port of Zara in Dalmatia (vassal city of Venice, which had rebelled and placed itself under Hungary's protection in 1186). Shortly afterward, Alexios Angelos, son of the deposed and blinded Emperor Isaac II Angelos, made contact with the crusaders. Alexios offered to reunite the Byzantine church with Rome, pay the crusaders 200,000 silver marks, join the crusade, and provide all the supplies they needed to reach Egypt.

The crusaders arrived at Constantinople in the summer of 1203 and quickly attacked, starting a major fire that damaged large parts of the city, and briefly seized control. Alexios III fled from the capital and Alexios Angelos was elevated to the throne as Alexios IV along with his blind father Isaac.

Alexios IV and Isaac II were unable to keep their promises and were deposed by Alexios V. The crusaders again took the city on 13 April 1204, and Constantinople was subjected to pillage and massacre by the rank and file for three days. Many priceless icons, relics, and other objects later turned up in Western Europe, a large number in Venice. According to Choniates, a prostitute was even set up on the Patriarchal throne.

After the sack of Constantinople in 1204 by Latin crusaders, two Byzantine successor states were established: the Empire of Nicaea, and the Despotate of Epirus. A third, the Empire of Trebizond, was created after Alexios Komnenos, commanding the Georgian expedition in Chaldia a few weeks before the sack of Constantinople, found himself de facto emperor, and established himself in Trebizond. Of the three successor states, Epirus and Nicaea stood the best chance of reclaiming Constantinople. The Nicaean Empire struggled to survive the next few decades, however, and by the mid-13th century, it had lost much of southern Anatolia.

The weakening of the Sultanate of Rûm following the Mongol invasion in 1242–43 allowed many beyliks and ghazis to set up their own principalities in Anatolia, weakening the Byzantine hold on Asia Minor.

The Empire of Nicaea, founded by the Laskarid dynasty, managed to effect the Recapture of Constantinople from the Latins in 1261 and defeat Epirus. This led to a short-lived revival of Byzantine fortunes under Michael VIII Palaiologos but the war-ravaged Empire was ill-equipped to deal with the enemies that surrounded it. To maintain his campaigns against the Latins, Michael pulled troops from Asia Minor and levied crippling taxes on the peasantry, causing much resentment. Rather than holding on to his possessions in Asia Minor, Michael chose to expand the Empire, gaining only short-term success. To avoid another sacking of the capital by the Latins, he forced the Church to submit to Rome, again a temporary solution for which the peasantry hated Michael and Constantinople.

The efforts of Andronikos II (1282 – 1328) and later his grandson Andronikos III (1328 – 1341) marked Byzantium's last genuine attempts in restoring the glory of the Empire. However, the use of mercenaries by Andronikos II often backfired, with the Catalan Company ravaging the countryside and increasing resentment towards Constantinople.

In time, one of the Beys, Osman I, created an empire that would eventually conquer Constantinople. However, the Mongol invasion also gave Nicaea a temporary respite from Seljuk attacks, allowing it to concentrate on the Latin Empire to its north.

The situation became worse for Byzantium during the civil wars after Andronikos III died. A six-year-long civil war devastated the empire, allowing the Serbian ruler Stefan Dušan (r. 1331–1346) to overrun most of the Empire's remaining territory and establish a Serbian Empire.

In 1354, an earthquake at Gallipoli devastated the fort, allowing the Ottomans (who were hired as mercenaries during the civil war by John VI Kantakouzenos) to establish themselves in Europe.

By the time the Byzantine civil wars had ended, the Ottomans had defeated the Serbians and subjugated them as vassals. Following the Battle of Kosovo, much of the Balkans became dominated by the Ottomans.

Constantinople by this stage was underpopulated and dilapidated. The population of the city had collapsed so severely that it was now little more than a cluster of villages separated by fields. On 2 April 1453, Sultan Mehmed's army of 80,000 men and large numbers of irregulars laid siege to the city.

Despite a desperate last-ditch defense of the city by the massively outnumbered forces (c. 7,000 men, 2,000 of whom were foreign), Constantinople finally fell to the Ottomans after a two-month siege on 29 May 1453. The final Byzantine emperor, Constantine XI Palaiologos, was last seen casting off his imperial regalia and throwing himself into hand-to-hand combat after the walls of the city were taken.