Latin Empire in the context of "Alexios IV Angelos"

Constantinople (see other names) was a historical city located on the Bosporus that served as the capital of the Roman (including its eastern continuation), Latin, and Ottoman empires between its consecration in 330 and the formal abolishment of the Ottoman sultanate in 1922. Initially, as New Rome, Constantinople was founded in 324 during the reign of Constantine the Great on the site of the existing settlement of Byzantium and in 330 became the capital of the Roman Empire. Following the collapse of the Western Roman Empire in the late 5th century, Constantinople remained the capital of the Eastern Roman Empire (also known as the Byzantine Empire; 330–1204 and 1261–1453), the Latin Empire (1204–1261), and the Ottoman Empire (1453–1922). In the aftermath of the Turkish War of Independence, the Turkish capital moved to Ankara. Although the city had been known as Istanbul since 1453, it was officially renamed Istanbul on 28 March 1930. As of October 2025, it is the most populous city in Europe, with a population of more than 16 million residents, straddling the Bosporus Strait and lying in both Europe and Asia, and is the financial center of Turkey.

In 324, following the reunification of the Eastern and Western Roman Empires, the ancient city of Byzantium was selected to serve as the new capital of the Roman Empire, and the city was renamed Nova Roma, or 'New Rome', by Emperor Constantine the Great. On 11 May 330, it was renamed Constantinople and dedicated to Constantine. Shortly afterward, in 474 AD, the Great Fire of Constantinople erupted and consequently devastated the region. Constantinople is generally considered to be the center and the "cradle of Orthodox Christian civilization." From the mid-5th century to the early 13th century, Constantinople was the largest and wealthiest city in Europe. The city became famous for its architectural masterpieces, such as Hagia Sophia, the cathedral of the Eastern Orthodox Church, which served as the seat of the Ecumenical Patriarchate; the sacred Imperial Palace, where the emperors lived; the Hippodrome; the Golden Gate of the Land Walls; and opulent aristocratic palaces. The University of Constantinople was founded in the 5th century and contained artistic and literary treasures before it was sacked in 1204 and 1453, including its vast Imperial Library. which contained more than 100,000 volumes. The city was the home of the Ecumenical Patriarch of Constantinople and guardian of Christendom's holiest relics, such as the Crown of Thorns and the True Cross.

The Second Bulgarian Empire (Middle Bulgarian: Ц(а)рьство бл(ъ)гарское; Modern Bulgarian: Второ българско царство, romanized: Vtoro Balgarsko Tsarstvo) was a medieval Bulgarian state that existed between 1185 and 1422. A successor to the First Bulgarian Empire, it reached the peak of its power under Tsars Kaloyan and Ivan Asen II before gradually being conquered by the Ottomans in the late 14th century.

Until 1256, the Second Bulgarian Empire was the dominant power in the Balkans, defeating the Byzantine Empire in several major battles. In 1205, Emperor Kaloyan defeated the newly established Latin Empire in the Battle of Adrianople. His nephew Ivan Asen II defeated the Despotate of Epiros and made Bulgaria a regional power again. During his reign, Bulgaria spread from the Adriatic to the Black Sea and the economy flourished. In the late 13th century, however, the Empire declined under constant invasions by Mongols, Byzantines, Hungarians, and Serbs, as well as internal unrest and revolts. The 14th century saw a temporary recovery and stability, but also the peak of Balkan feudalism as central authorities gradually lost power in many regions. Bulgaria was divided into three parts on the eve of the Ottoman invasion.

Istanbul is the largest city in Turkey, constituting the country's economic, cultural, and historical heart. With a population over 15 million, it is home to 18% of the population of Turkey. Istanbul is among the largest cities in Europe and in the world by population. It is a city on two continents; about two-thirds of its population live in Europe and the rest in Asia. Istanbul straddles the Bosphorus—one of the world's busiest waterways—in northwestern Turkey, between the Sea of Marmara and the Black Sea. Its area of 5,461 square kilometers (2,109 sq mi) is coterminous with Istanbul Province. Under the Köppen climate classification, Istanbul's climate is considered a form of Mediterranean climate, with aspects of other temperate climate types.

The city now known as Istanbul developed to become one of the most significant cities in history. Byzantium was founded on the Sarayburnu promontory by Greek colonists, potentially in the seventh century BC. Over nearly 16 centuries following its reestablishment as Constantinople in 330 AD, it served as the capital of four empires: the Roman Empire (330–395), the Byzantine Empire (395–1204 and 1261–1453), the Latin Empire (1204–1261), and the Ottoman Empire (1453–1922). It was instrumental in the advancement of Christianity during Roman and Byzantine times, before the Ottomans conquered the city in 1453 and transformed it into an Islamic stronghold and the seat of the last caliphate. Although the Republic of Turkey established its capital in Ankara, palaces and imperial mosques still line Istanbul's hills as visible reminders of the city's previous central role. The historic centre of Istanbul is a UNESCO World Heritage Site.

The High Middle Ages, or High Medieval Period, was the period of European history between c. 1000 and c. 1300; it was preceded by the Early Middle Ages and followed by the Late Middle Ages, which ended c. 1500 according to historiographical convention. Key historical trends of the High Middle Ages include the rapidly increasing population of Europe, which brought about great social and political change from the preceding era, and the Renaissance of the 12th century, including the first developments of the rural exodus and urbanization. By 1350, the robust population increase had greatly benefited the European economy, which had reached levels that would not be seen again in some areas until the 19th century. That trend faltered in the early 14th century, as the result of numerous events which together comprised the crisis of the late Middle Ages—most notable among them being the Black Death, in addition to various regional wars and economic stagnation.

From c. 780, Europe saw the last of the barbarian invasions and became more socially and politically organized. The Carolingian Renaissance stimulated scientific and philosophical activity in Northern Europe. The first universities started operating in Bologna, Oxford, Paris, Salamanca, Cambridge and Modena. The Vikings settled in the British Isles, France and elsewhere, and Norse Christian kingdoms started developing in their Scandinavian homelands. The Magyars ceased their expansion in the 10th century, and by 1000, a Christian Kingdom of Hungary had become a recognized state in Central Europe that was forming alliances with regional powers. With the brief exception of the Mongol invasions in the 13th century, major nomadic incursions ceased. The powerful Byzantine Empire of the Macedonian and Komnenos dynasties gradually gave way to the resurrected Serbia and Bulgaria and to a successor crusader state (1204 to 1261), who continually fought each other until the end of the Latin Empire. The Byzantine Empire was reestablished in 1261 with the recapture of Constantinople from the Latins, though it was no longer a major power and would continue to falter through the 14th century, with remnants lasting until the mid 15th century.

Hagia Sophia, officially the Hagia Sophia Grand Mosque, is a mosque serving as a major cultural and historical site in Istanbul, Turkey. It was formerly a church (360–1453) and a museum (1935–2020). The last of three church buildings to be successively erected on the site by the Eastern Roman Empire, it was completed in AD 537, becoming the world's largest interior space and among the first to employ a fully pendentive dome. It is considered the epitome of Byzantine architecture and is said to have "changed the history of architecture". From its dedication in 360 until 1453 Hagia Sophia served as the cathedral of Constantinople in the Byzantine liturgical tradition, except for the period 1204–1261 when the Latin Crusaders installed their own hierarchy. After the fall of Constantinople in 1453, it served as a mosque, having its minarets added soon after. The site became a museum in 1935, and was redesignated as a mosque in 2020.

The current structure was built by the Byzantine emperor Justinian I as the Christian cathedral of Constantinople between 532–537 and was designed by the Greek geometers Isidore of Miletus and Anthemius of Tralles. It was formally called the Temple of God's Holy Wisdom, (Greek: Ναὸς τῆς Ἁγίας τοῦ Θεοῦ Σοφίας, romanized: Naòs tês Hagías toû Theoû Sophías) the third church of the same name to occupy the site, as the prior one had been destroyed in the Nika riots. As the episcopal see of the ecumenical patriarch of Constantinople, it remained the world's largest cathedral for nearly a thousand years, until the Seville Cathedral was completed in 1520.

The sack of Constantinople occurred in April 1204 and marked the culmination of the Fourth Crusade. Crusaders sacked and destroyed most of Constantinople, the capital of the Byzantine Empire. After the capture of the city, the Latin Empire (known to the Byzantines as the Frankokratia, or the Latin occupation) was established and Baldwin IX of Flanders crowned emperor in Hagia Sophia.

After the city's sacking, most of the Byzantine Empire's territories were divided up among the Crusaders. Byzantine aristocrats also established a number of small independent splinter states—one of them being the Empire of Nicaea, which eventually recaptured Constantinople in 1261 and proclaimed the reinstatement of the Empire. However, the restored Empire never managed to reclaim all its former territory or attain its earlier economic strength, and it gradually succumbed to the rising Ottoman Empire over the following two centuries.

The Empire of Nicaea (Greek: Βασιλεία Ῥωμαίων), also known as the Nicene Empire, was the largest of the three Byzantine Greek rump states founded by the aristocracy of the Byzantine Empire that fled when Constantinople was occupied by Western European and Venetian armed forces during the Fourth Crusade, a military event known as the Sack of Constantinople. Like the other Byzantine rump states that formed due to the 1204 fracturing of the empire, such as the Empire of Trebizond and the Despotate of Epirus, it was a continuation of the eastern half of the Roman Empire that survived well into the Middle Ages. A fourth state, known in historiography as the Latin Empire, was established by an army of Crusaders and the Republic of Venice after the capture of Constantinople and the surrounding environs.

Founded by the Laskaris family, it lasted from 1204 to 1261, when the Nicenes restored the Byzantine Empire after they recaptured Constantinople. Thus, the Nicene Empire is seen as a direct continuation of the Byzantine Empire, as it fully assumed the traditional titles and government of the Byzantines in 1205.

The Byzantine–Ottoman wars were a series of decisive conflicts between the Byzantine Greeks and Ottoman Turks and their allies that led to the final destruction of the Byzantine Empire and the rise of the Ottoman Empire. The Byzantines, already having been in a weak state even before the partitioning of their Empire following the 4th Crusade, failed to recover fully under the rule of the Palaiologos dynasty. Thus, the Byzantines faced increasingly disastrous defeats at the hands of the Ottomans. Ultimately, they lost Constantinople in 1453, formally ending the conflicts (however, several Byzantine Holdouts lasted until 1479).

Taking advantage of the situation, the Seljuk Sultanate of Rum began seizing territory in western Anatolia, until the Nicaean Empire was able to repulse the Seljuk Turks from the remaining territories still under Byzantine rule. Eventually Constantinople was re-taken from the Latin Empire in 1261 by the Nicaean Empire. The position of the Byzantine Empire in Europe remained uncertain due to the presence of the rivals in Epirus, Serbia and Bulgaria. This, combined with the declining power of the Sultanate of Rum (Byzantium's chief rival in Asia Minor) led to the removal of troops from Anatolia to maintain Byzantium's grip on Thrace.

The Empire of Thessalonica is a historiographic term used by some modern scholars to refer to the short-lived Byzantine Greek state centred on the city of Thessalonica between 1224 and 1246 (sensu stricto until 1242) and ruled by the Komnenodoukas dynasty of Epirus. At the time of its establishment during the struggle for Constantinople, the Empire of Thessalonica, under the capable Theodore Komnenos Doukas, rivaled the Empire of Nicaea and the Second Bulgarian Empire as the strongest state in the region, and aspired to capturing Constantinople, putting an end to the Latin Empire, and restoring the Byzantine Empire that had been extinguished in 1204.

Thessalonica's ascendancy was brief, ending with the disastrous Battle of Klokotnitsa against Bulgaria in 1230, where Theodore Komnenos Doukas was captured. Reduced to a Bulgarian vassal, Theodore's brother and successor Manuel Komnenos Doukas was unable to prevent the loss of most of his brother's conquests in Macedonia and Thrace, while the original nucleus of the state, Epirus, broke free under Michael II Komnenos Doukas. Theodore recovered Thessalonica in 1237, installing his son John Komnenos Doukas, and after him Demetrios Angelos Doukas, as rulers of the city, while Manuel, with Nicaean support, seized Thessaly. The rulers of Thessalonica bore the imperial title from 1225/7 until 1242, when they were forced to renounce it and recognize the suzerainty of the rival Empire of Nicaea. The Komnenodoukai continued to rule as Despots of Thessalonica for four more years after that, but in 1246 the city was annexed by Nicaea.