 preference since he supported it with generous privileges.
Theodosius the Great made Christianity the official state religion of the Roman Empire with the Edict of Thessalonica in 380 and was instrumental in establishing the Nicene Creed as the orthodox doctrine for Christianity with the First Council of Constantinople in 381.
Following the death of Theodosius the Great in 395 and the permanent division of the Roman Empire between his two sons, Constantinople became the capital of the Eastern Roman Empire. This empire, which would later be branded by historians as the Byzantine Empire, ruled most of the territory of present-day Turkey until the Late Middle Ages; although the eastern regions remained firmly in Sasanian hands until the 7th century. The frequent Byzantine-Sassanid Wars, a continuation of the centuries-long Roman-Persian Wars, took place between the 4th and 7th centuries.
Several ecumenical councils of the early Church were held in cities located in present-day Turkey, including the First Council of Nicaea (Iznik) in 325 (which resulted in the first uniform Christian doctrine, called the Nicene Creed), the First Council of Constantinople (Istanbul) in 381, the Council of Ephesus in 431, and the Council of Chalcedon (Kadıköy) in 451. During most of its existence, the Byzantine Empire was one of the most powerful economic, cultural, and military forces in Europe. Established in the Roman period, the Ecumenical Patriarchate of Constantinople is the oldest continuously active institution in Istanbul. The First Council of Constantinople in 381 recognized that the rights of the bishop of Constantinople are equal to those of the bishop of Rome.
Great Seljuk Empire.
The House of Seljuk originated from the "Kınık" branch of the Oghuz Turks who resided in the Yabgu Khaganate, on the periphery of the Muslim world, in the 9th century. In the 10th century, the Seljuks started migrating from their ancestral homeland into Persia, which became the administrative core of the Great Seljuk Empire, after its foundation by Tughril.
In the latter half of the 11th century, the Seljuk Turks began penetrating into medieval Armenia and Anatolia. In 1071, the Seljuks defeated the Byzantines at the Battle of Manzikert, starting the Turkification process in the area; the Turkish language and Islam were introduced to Anatolia. The slow transition from a predominantly Christian and Greek-speaking Anatolia to a predominantly Muslim and Turkish-speaking one was underway.
The Mevlevi Order of dervishes, established in Konya during the 13th century by Sufi poet Mevlânâ Rûmî, played a role in the Islamization of the diverse people of Anatolia. Thus, alongside the Turkification of the territory, the culturally Persianized Seljuks set the basis for a Turko-Persian principal culture in Anatolia.
The defeat of the Seljuk armies by the Mongols in 1243 caused the territories of the Seljuk Sultanate of Rûm (Anatolia) to slowly disintegrate into small Turkish principalities.
Ottoman Empire.
In the early 14th century, the Ottoman Beylik founded by Osman I started expanding its territory and annexing the nearby Turkish beyliks (principalities) in Anatolia. Within a few decades, during the reign of Murad I (r. 1362–1389), the Ottoman State began expanding into the Balkans, eventually becoming known as the Ottoman Empire. The Ottomans completed their conquest of the Byzantine Empire by capturing its capital, Constantinople, on 29 May 1453: their sultan and commander-in-chief Mehmed II thenceforth being known as Mehmed the Conqueror. Mehmed II further expanded the territories of the Ottoman Empire in Anatolia and the Balkan peninsula. His expedition to Italy (1480–1481), commanded by Gedik Ahmed Pasha, began with the Ottoman invasion of Otranto and the nearby areas in Apulia. The invasion, which had the goal of establishing a foothold on the Italian peninsula for a subsequent conquest of Rome, started on 28 July 1480 and ended on 10 September 1481, four months after Mehmed II's death on 3 May 1481.
Following the end of the Reconquista, which resulted in the expulsion of non-Christians (Jews and Muslims) from Iberia and southern Italy controlled by the Crowns of Castile and Aragon (and later by the Spanish Empire), a large number of Sephardic Jews and Andalusian Muslims emigrated to the Ottoman Empire during the reigns of Sultan Bayezid II and his successors, settling primarily in Istanbul, Izmir, Selanik, Bursa and Edirne.
In 1514, Sultan Selim I (1512–1520) successfully expanded the empire's borders by defeating Shah Ismail I of the Safavid dynasty in the Battle of Chaldiran. In 1517, Selim I expanded Ottoman rule into Algeria and Egypt, and created a naval presence in the Red Sea. Subsequently, a contest started between the Ottoman and Portuguese empires to become the dominant sea power in the Indian Ocean, with a number of naval battles in the Red Sea, the Arabian Sea and the Persian Gulf. The Portuguese presence in the Indian Ocean was perceived as a threat to the Ottoman monopoly over the ancient trade routes between East Asia and Western Europe. Despite the increasingly prominent European presence, the Ottoman Empire's trade with the east continued to flourish until the second half of the 18th century.
The Ottoman Empire's power and prestige peaked in the 16th and 17th centuries, particularly during the reign of Suleiman the Magnificent, who personally instituted major legislative changes relating to society, education, taxation and criminal law.
The empire was often at odds with the Holy Roman Empire in its steady advance towards Central Europe through the Balkans and the southern part of the Polish–Lithuanian Commonwealth.
The Ottoman Navy contended with several Holy Leagues, such as those in 1538, 1571, 1684 and 1717 (composed primarily of Habsburg Spain, the Republic of Genoa, the Republic of Venice, the Knights of St. John, the Papal States, the Grand Duchy of Tuscany and the Duchy of Savoy), for the control of the Mediterranean Sea.
In the east, the Ottomans were often at war with Safavid Persia over conflicts between the 16th and 18th centuries. The Ottoman wars with Persia continued as the Zand, Afsharid, and Qajar dynasties succeeded the Safavids in Iran, until the first half of the 19th century.
Even further east, there was an extension of the Habsburg-Ottoman conflict, in that the Ottomans also had to send soldiers to their farthest and easternmost vassal and territory, the Aceh Sultanate in Southeast Asia, to defend it from European colonizers as well as the Latino invaders who had crossed from Latin America and had Christianized the formerly Muslim-dominated Philippines.
From the 16th to the 20th centuries, the Ottoman Empire also fought twelve wars with the Russian Tsardom and Empire. These were initially about Ottoman territorial expansion and consolidation in southeastern and eastern Europe; but starting from the Russo-Turkish War (1768–1774), they became more about the survival of the Ottoman Empire, which had begun to lose its strategic territories on the northern Black Sea coast to the advancing Russians.
From the second half of the 18th century onwards, the Ottoman Empire began to decline. The Tanzimat reforms, initiated by Mahmud II in 1839, aimed to modernize the Ottoman state in line with the progress that had been made in Western Europe. The efforts of Midhat Pasha during the late Tanzimat era led the Ottoman constitutional movement of 1876, which introduced the First Constitutional Era, but these efforts proved to be inadequate in most fields, and failed to stop the dissolution of the empire.
As the empire gradually shrank in size, military power and wealth; especially after the Ottoman economic crisis and default in 1875 which led to uprisings in the Balkan provinces that culminated in the Russo-Turkish War (1877–1878); many Balkan Muslims migrated to the Empire's heartland in Anatolia, along with the Circassians fleeing the Russian conquest of the Caucasus. According to some estimates, 800,000 Muslim Circassians died during the Circassian genocide in the territory of present-day Russia, the survivors of which sought refuge in the Ottoman Empire, mostly settling in the provinces of present-day Turkey. The decline of the Ottoman Empire led to a rise in nationalist sentiment among its various subject peoples, leading to increased ethnic tensions which occasionally burst into violence, such as the Hamidian massacres of Armenians, which claimed up to 300,000 lives.
The loss of Rumelia (Ottoman territories in Europe) with the First Balkan War (1912–1913) was followed by the arrival of millions of Muslim refugees ("muhacir") to Istanbul and Anatolia. Historically, the Rumelia Eyalet and Anatolia Eyalet had formed the administrative core of the Ottoman Empire, with their governors titled Beylerbeyi participating in the Sultan's Divan, so the loss of all Balkan provinces beyond the Midye-Enez border line according to the London Conference of 1912–13 and the Treaty of London (1913) was a major shock for the Ottoman society and led to the 1913 Ottoman coup d'