Emperor of the Roman Empire

The Roman emperor was the ruler of the Roman Empire during the imperial period (starting in 27 BC). The emperors used a variety of different titles throughout history. Often when a given Roman is described as becoming "emperor" in English, it reflects his taking of the title Augustus or Caesar. Another title often used was imperator, originally a military honorific. Early Emperors also used the title Princeps Civitatis ('first citizen'). Emperors frequently amassed republican titles, notably princeps senatus, consul and pontifex maximus. The legitimacy of an emperor's rule depended on his control of the army and recognition by the Senate; an emperor would normally be proclaimed by his troops, or invested with imperial titles by the Senate, or both. The first emperors reigned alone; later emperors would sometimes rule with co-emperors and divide administration of the empire between them. The Romans considered the office of emperor to be distinct from that of a king. The first emperor, Augustus, resolutely refused recognition as a monarch. Although Augustus could claim that his power was authentically republican, his successors, Tiberius and Nero, could not convincingly make the same claim. Nonetheless, for the first three hundred years of Roman emperors, from Augustus until Diocletian, efforts were made to portray the emperors as leaders of a republic.

From Diocletian, whose tetrarchic reforms also divided the position into one emperor in the West and one in the East, until the end of the Empire, emperors ruled in an openly monarchic style[3] and did not preserve the nominal principle of a republic, but the contrast with "kings" was maintained: although the imperial succession was generally hereditary, it was only hereditary if there was a suitable candidate acceptable to the army and the bureaucracy, so the principle of automatic inheritance was not adopted. Elements of the republican institutional framework (senate, consuls, and magistrates) were preserved even after the end of the Western Empire. The peaceful reign of Constantine the Great, the first to openly convert to Christianity and allowing freedom of religion, witnessed the replacement of the Caput Mundi from Rome to Constantinople. The Western Roman Empire collapsed in the late 5th century after multiple invasions of imperial territory by Germanic barbarian tribes. Romulus Augustulus is often considered to be the last emperor of the West after his forced abdication in 476, although Julius Nepos maintained a claim recognized by the Eastern Empire to the title until his death in 480. Following Nepos' death, the Eastern Emperor Zeno abolished the division of the position and proclaimed himself as the sole Emperor of a reunited Roman Empire. Emperor Heraclius made diplomatic relations with the Companions of the Prophet Muhammad, but lost many territories after successfull Islamic conquests. The Eastern imperial lineage continued to rule from Constantinople ("New Rome"); they continued to style themselves as Emperor of the Romans (later βασιλεύς Ῥωμαίων in Greek), but are often referred to in modern scholarship as Byzantine emperors. Constantine XI Palaiologos was the last Roman emperor in Constantinople, dying in the Fall of Constantinople to the Ottoman Empire's Mehmed II in 1453. The Muslim rulers then claimed the title of Caesar of Rome. The "Byzantine" emperors from Heraclius in 629 and onwards adopted the title of basileus (βασιλεύς), which had originally meant king in Greek but became a title reserved solely for the Roman emperor and the ruler of the Sasanian Empire. Other kings were then referred to as rēgas. In addition to their pontifical office, some emperors were given divine status after death. With the eventual hegemony of Christianity, the emperor came to be seen as God's chosen ruler, as well as a special protector and leader of the Christian Church on Earth, although in practice an emperor's authority on Church matters was subject to challenge.Due to the cultural rupture of the Turkish conquest, most western historians treat Constantine XI as the last meaningful claimant to the title Roman Emperor. From 1453, one of the titles used by the Ottoman Sultans was "Caesar of Rome" (Turkish: Kayser-i Rum), part of their titles until the Ottoman Empire ended in 1922. A Byzantine group of claimant Roman emperors existed in the Empire of Trebizond until its conquest by the Ottomans in 1461, though they had used a modified title since 1282.