The Western Roman Empire collapsed in the late 5th century after multiple invasions by Germanic barbarian tribes, with no recognised claimant to Emperor of the West remaining after the death of Julius Nepos in 480.

[6] Conversely, the majority of Roman writers, including Pliny the Younger, Suetonius and Appian, as well as most of the ordinary people of the Empire, thought of Julius Caesar as the first emperor.

His killers proclaimed themselves as the liberatores ("liberators") and the restorers of the Republic, but their rule was cut short by Caesar's supporters, who almost immediately established a new dictatorship.

He inherited his property and lineage, the loyalty of most of his allies, and – again through a formal process of senatorial consent – an increasing number of the titles and offices that had accrued to Caesar.

The Senate confirmed Octavian as princeps, the "first among equals", and gave him control over almost all Roman provinces for a tenure of ten years.

The honorific itself held no legal meaning, but it denoted that Octavian (henceforth Augustus) now approached divinity, and its adoption by his successors made it the de facto main title of the emperor.

Tiberius had already received imperium maius and tribunicia potestas in AD 4, becoming legally equal to Augustus but still subordinate to him in practice.

[13] The "imperial office" was thus not truly defined until the accession of Caligula, when all of Tiberius' powers were automatically transferred to him as a single, abstract position that was symbolized by his sacred title of augustus.

[29] The only surviving document to directly refer to the emperor's power is the Lex de imperio Vespasiani, written shortly after Vespasian's formal accession in December 69.

[35] Formally announcing a successor would have revealed Augustus as a monarch, so he and subsequent emperors opted to adopt their best candidates as their sons and heirs.

Primogeniture was not relevant in the early Empire, although emperors still attempted to maintain a familiar connection between them; Tiberius, for example, married Julia the Elder, making him Augustus' son-in-law.

[36] The Senate still exercised some power during this period, as evidenced by his decision to declare Nero a "public enemy",[37] and did influence in the succession of emperors.

[39][d] Despite being the son of a previous emperor and having nominally shared government with him, Commodus' rule ended with his murder at the hands of his own soldiers.

The Lex de imperio Vespasiani explicitly states that all of Vespasian's actions are considered legal even if they happened before his recognition by the Senate.

[45] In reality, Licinius was the legitimate emperor of the West (having been appointed by Galerius), while Constantine was the real "usurper" (having been proclaimed by his troops).

[60] Beginning in the late 2nd century, the Empire began to suffer a series of political and economic crises, partially because it had overexpanded so much.

Reaching back to the oldest traditions of job-sharing in the Republic, Diocletian established at the top of this new structure the Tetrarchy ("rule of four") in an attempt to provide for smoother succession and greater continuity of government.

Constantine I, the son of tetrarch Constantius I, reunited the empire in 324 and imposed the principle of hereditary succession which Diocletian intended to avoid.

[83] In 476, the Heruli Odoacer overthrew the child-emperor Romulus Augustulus, made himself king of Italy and shipped the imperial regalia to the Emperor Zeno in Constantinople.

Africa was lost to the Arabs in the early 7th century, and Rome eventually fell to the Lombards in 751, during the reign of Constantine V. The Frankish king Pepin the Short defeated them and received the favour of Pope Stephen II, who became the head of the Papal States.

Starting with Heraclius in 629, Roman emperors styled themselves "basileus", the traditional title for Greek monarchs used since the times of Alexander the Great.

[94] By the times of the Palaiologos, there were two distinct ceremonies for the accession of an emperor: first an acclamation as basileus, and later a coronation as autokrator (which also included being raised on a shield).

[97] The Byzantine state is often said to have followed a "Caesaropapist" model, where the emperor played the role of ruler and head of the Church, but there was often a clear distinction between political and secular power.

Five days before his murder he adopted Piso Licinianus as his son and heir, renaming him as Servius Sulpicius Galba Caesar.

[117] The last use of the formula, rendered as Autokrator Kaisar Flabios... Augoustos (Αὐτοκράτωρ καῖσαρ Φλάβιος αὐγουστος) in Greek, is in the Basilika of Leo VI the Wise (r.

[119] According to Suetonius, it was "not merely a new title but a more honorable one, inasmuch as sacred places too, and those in which anything is consecrated by augural rites are called "august" (augusta), from the increase (auctus) in dignity".

[n] Despite its use as the highest imperial title, it was generally not used to indicate the office of Emperor itself, as ordinary people and writers had become accustomed to Imperator.

[122] The word princeps, meaning "first", was a republican term used to denote the leading member of the Senate, and it was used by the early emperors to emphasize the continuance of the Republic.

"[135] The last attempt to restore the office of emperor in the West was during the Siege of Ravenna (539–540), when the Goths offered Belisarius the throne, which he refused.

The 4th-century historian Festus states that "From Octavian Caesar Augustus to Jovian, there were imperatores, 43 in number, through 407 years [reckoning from 43 BC]".