Caesar (title) in the context of "Constantius Chlorus"

In modern historiography, the Western Roman Empire were the Roman Empire's western provinces, collectively, during any period in which they were administered separately from the eastern provinces by a separate, independent imperial court. Particularly during the period from AD 395 to 476, there were separate, coequal courts dividing the governance of the empire into the Western provinces and the Eastern provinces with a distinct imperial succession in the separate courts. The terms Western Roman Empire and Eastern Roman Empire were coined in modern times to describe political entities that were de facto independent; contemporary Romans did not consider the Empire to have been split into two empires but viewed it as a single polity governed by two imperial courts for administrative expediency. The Western Empire collapsed in 476, and the Western imperial court in Ravenna disappeared by 554, at the end of Justinian's Gothic War.

Though there were periods with more than one emperor ruling jointly before, the view that it was impossible for a single emperor to govern the entire Empire was institutionalized by emperor Diocletian following the disastrous civil wars and disintegrations of the Crisis of the Third Century. He introduced the system of the Tetrarchy in 286, with two senior emperors titled Augustus, one in the East and one in the West, each with an appointed subordinate and heir titled Caesar. Though the tetrarchic system would collapse in a matter of years, the East–West administrative division would endure in one form or another over the coming centuries. As such, the unofficial Western Roman Empire would exist intermittently in several periods between the 3rd and 5th centuries. Some emperors, such as Constantine I and Theodosius I, governed, if briefly, as the sole Augustus across the Roman Empire. On the death of Theodosius in 395, the empire was divided between his two infant sons, with Honorius as his successor in the West governing briefly from Mediolanum then from Ravenna, and Arcadius as his successor in the East governing from Constantinople.

Mehmed II (Ottoman Turkish: محمد ثانى, romanized: Meḥemmed-i s̱ānī; Turkish: II. Mehmed, pronounced [icinˈdʒi ˈmehmet]; 30 March 1432 – 3 May 1481), commonly known as Mehmed the Conqueror (Ottoman Turkish: ابو الفتح, romanized: Ebū'l-fetḥ, lit. 'the Father of Conquest'; Turkish: Fâtih Sultan Mehmed), was twice the sultan of the Ottoman Empire from August 1444 to September 1446 and then later from February 1451 to May 1481.

In Mehmed II's first reign, he defeated the crusade led by John Hunyadi after the Hungarian incursions into his country broke the conditions of the truce per the Treaties of Edirne and Szeged. When Mehmed II ascended the throne again in 1451, he strengthened the Ottoman Navy and made preparations to attack Constantinople. At the age of 21, he conquered Constantinople and brought an end to the Byzantine Empire. After the conquest, Mehmed claimed the title caesar of Rome (Ottoman Turkish: قیصر روم, romanized: qayṣar-i rūm), based on the fact that Constantinople had been the seat and capital of the surviving Eastern Roman Empire since its consecration in 330 AD by Emperor Constantine I. The claim was soon recognized by the Patriarchate of Constantinople, albeit not by most European monarchs.

The Roman emperor was the ruler and monarchical head of state of the Roman Empire, starting with the granting of the title augustus to Octavian in 27 BC. The title of imperator, originally a military honorific, was usually used alongside caesar, originally a cognomen. When a given Roman is described as becoming emperor in English, it generally reflects his accession as augustus, and later as basileus. Early emperors also used the title princeps ("first one") alongside other Republican titles, notably consul and pontifex maximus.

The legitimacy of an emperor's rule depended on his control of the Roman army and recognition by the Senate; an emperor would normally be proclaimed by his troops, or by the Senate, or both. The first emperors reigned alone; later emperors would sometimes rule with co-emperors to secure the succession or to divide the administration of the empire between them. The office of emperor was thought to be distinct from that of a rex ("king"). Augustus, the first emperor, resolutely refused recognition as a monarch. For the first three hundred years of Roman emperors, efforts were made to portray the emperors as leaders of the Republic, fearing any association with the kings who ruled Rome prior to the Republic.

Diocletian (/ˌdaɪ.əˈkliːʃən/ DY-ə-KLEE-shən; Latin: Gaius Aurelius Valerius Diocletianus; Ancient Greek: Διοκλητιανός, romanized: Diokletianós; 242/245 – 311/312), nicknamed Jovius, was Roman emperor from 284 until his abdication in 305. He was born Diocles to a family of low status in the Roman province of Dalmatia. As with other Illyrian soldiers of the period, Diocles rose through the ranks of the military early in his career, serving under Aurelian and Probus, and eventually becoming a cavalry commander for the army of Emperor Carus. After the deaths of Carus and his son Numerian on a campaign in Persia, Diocles was proclaimed emperor by the troops, taking the name "Diocletianus". The title was also claimed by Carus's surviving son, Carinus, but Diocletian defeated him in the Battle of the Margus.

Diocletian's reign stabilized the empire and ended the Crisis of the Third Century. He initiated the process of the Roman Empire split and appointed fellow officer Maximian as Augustus, co-emperor, in 286. Diocletian reigned in the Eastern Empire, and Maximian reigned in the Western Empire. Diocletian delegated further on 1 March 293, appointing Galerius and Constantius as junior colleagues (each with the title Caesar), under himself and Maximian respectively. Under the Tetrarchy, or "rule of four", each tetrarch would rule over a quarter-division of the empire. Diocletian secured the empire's borders and purged it of all threats to his power. He defeated the Sarmatians and Carpi during several campaigns between 285 and 299, the Alamanni in 288, and usurpers in Egypt between 297 and 298. Galerius, aided by Diocletian, campaigned successfully against Persia, the empire's traditional enemy, and in 299, he sacked their capital, Ctesiphon. Diocletian led the subsequent negotiations and achieved a lasting and favorable peace.

The Leonid dynasty or Thracian dynasty produced six Byzantine emperors during Late Antiquity, reigning over the Byzantine Empire from 457 to 518. The dynasty's patriarch was Leo I, who was made Byzantine emperor in 457. Leo's daughter Ariadne became empress and mother to an emperor, and her two husbands were themselves each made emperor in turn. Another relative whose name does not survive of Leo I or his wife Verina married the future augustus Julius Nepos (r. 474–480), the last emperor in the Western Roman Empire. The dynasty of Leo succeeded the preceding Valentinianic dynasty (r. 364–455) and Theodosian dynasty (r. 379–457) whose family trees were conjoined and ruled concurrently. Besides Julius Nepos, who administered no more than a rump state the Roman province of Dalmatia in the western empire during the fall of the west, the dynasty's emperors governed the Byzantine Empire (east).

Leo's eldest daughter Ariadne married Zeno, and their son, Leo II was proclaimed augustus and succeeded his grandfather Leo as an infant, appointing Zeno co-augustus. Leo's younger daughter, Leontia, married first the caesar Patricius, son of the magister militum Aspar, and then Marcianus, son of the augustus Anthemius (r. 467–472). When Leo II died in the year of his grandfather's death, Zeno remained augustus in the Byzantine Empire.

Béla III (Hungarian: III. Béla, Croatian: Bela III., Slovak: Belo III.; c. 1148 – 23 April 1196) was King of Hungary and Croatia between 1172 and 1196. He was the second son of King Géza II and Géza's wife, Euphrosyne of Kiev. Around 1161, Géza granted Béla a duchy, which included Croatia, central Dalmatia and possibly Sirmium. In accordance with a peace treaty between his elder brother, Stephen III, who succeeded their father in 1162, and the Byzantine Emperor Manuel I Komnenos, Béla moved to Constantinople in 1163. He was renamed to Alexios, and the emperor granted him the newly created senior court title of despotes. He was betrothed to the Emperor's daughter, Maria. Béla's patrimony caused armed conflicts between the Byzantine Empire and the Kingdom of Hungary between 1164 and 1167, because Stephen III attempted to hinder the Byzantines from taking control of Croatia, Dalmatia and Sirmium. Béla-Alexios, who was designated as Emperor Manuel's heir in 1165, took part in three Byzantine campaigns against Hungary. His betrothal to the emperor's daughter was dissolved after her brother, Alexios, was born in 1169. The emperor deprived Béla of his high title, granting him the inferior rank of kaisar.

Stephen III died on 4 March 1172, and Béla decided to return to Hungary. Before his departure, he pledged that he would never make war against the Byzantine Empire. Although the Hungarian prelates and lords unanimously proclaimed Béla king, Lucas, Archbishop of Esztergom opposed his coronation because of Béla's alleged simony. Finally, the Archbishop of Kalocsa crowned him king on 18 January 1173, with Pope Alexander III's approval. Béla fought with his younger brother, Géza, whom he held in captivity for more than a decade. Taking advantage of the internal conflicts in the Byzantine Empire after Emperor Manuel's death, Béla reoccupied Croatia, Dalmatia and Sirmium between 1180 and 1181. He occupied the Principality of Halych in 1188, but it was lost within two years.

Tsar (/zɑːr, (t)sɑːr/; also spelled czar, tzar, or csar; Bulgarian: цар, romanized: tsar; Russian: царь, romanized: tsar'; Serbian: цар, car) is a title historically used by Slavic monarchs. The term is derived from the Latin word caesar, which was intended to mean emperor in the European medieval sense of the term—a ruler with the same rank as a Roman emperor, holding it by the approval of another emperor or a supreme ecclesiastical official—but was usually considered by Western Europeans to be equivalent to "king".

Tsar and its variants were the official titles in the First Bulgarian Empire (681–1018), Second Bulgarian Empire (1185–1396), the Kingdom of Bulgaria (1908–1946), the Serbian Empire (1346–1371), and the Tsardom of Russia (1547–1721).The first ruler to adopt the title tsar was Simeon I of Bulgaria. Simeon II, the last tsar of Bulgaria, is the last person to have held this title.