Ancient Rome became a territorial empire while still a republic, but was then ruled by emperors beginning with Octavian Augustus, the final victor of the republican civil wars.

Despite this gesture of universality, the Severan dynasty was tumultuous—an emperor's reign was ended routinely by his murder or execution—and following its collapse, the Empire was engulfed by the Crisis of the Third Century, a 50-year period of invasions, civil strife, economic disorder, and epidemic disease.

The Senate granted him power over appointing its membership and several successive consulships, allowing him to operate within the existing constitutional machinery and thus reject titles that Romans associated with monarchy, such as rex ("king").

Between June 68 and December 69, Rome witnessed the successive rise and fall of Galba, Otho and Vitellius until the final accession of Vespasian, first ruler of the Flavian dynasty.

Titus, Vespasian's son and successor, quickly proved his merit, although his short reign was marked by disaster, including the eruption of Mount Vesuvius in Pompeii.

Following the suicide of Otho, Vespasian was able to take control of Rome's winter grain supply in Egypt, placing him in a good position to defeat his remaining rival, Vitellius.

In the west he gave considerable favouritism to Hispania (the Iberian Peninsula, comprising modern Spain and Portugal) in which he granted Latin Rights to over three hundred towns and cities, promoting a new era of urbanisation throughout the western (formerly barbarian) provinces.

He had served as an effective general under his father, helping to secure the east and eventually taking over the command of Roman armies in Syria and Iudaea, quelling a significant First Jewish–Roman War at the time.

Although there was some trepidation when he took office because of his known dealings with some of the less respectable elements of Roman society, he quickly proved his merit, even recalling many exiled by his father as a show of good faith.

This led to his murder in 96, orchestrated by his enemies in the Senate, Stephanus (the steward of the deceased Julia Flavia), members of the Praetorian Guard and the empress Domitia Longina.

Support for Domitian in the army remained strong, and in October 97 the Praetorian Guard laid siege to the Imperial Palace on the Palatine Hill and took Nerva hostage.

Upon his accession to the throne, Trajan prepared and launched a carefully planned military invasion in Dacia, a region north of the lower Danube whose inhabitants the Dacians had long been an opponent to Rome.

Then he turned south into Parthian territory in Mesopotamia, taking the cities of Babylon, Seleucia and finally the capital of Ctesiphon in 116, while suppressing the Kitos War, a Jewish uprising across the eastern provinces.

[34] These were preceded by the appearance of Roman glasswares in Chinese tombs, the earliest piece found at Guangzhou (along the coast of the South China Sea) and dating to the 1st century BC.

He attempted to emulate the restrained practices of Marcus Aurelius, and made an effort to reform the welfare programme for poor children but he faced antagonism from many quarters.

[44] In early March he narrowly averted one conspiracy by a group to replace him with the consul Quintus Pompeius Sosius Falco while he was in Ostia inspecting the arrangements for grain shipments.

[56] When news of the public anger in Rome spread across the Empire, the generals Pescennius Niger in Syria, Septimius Severus in Pannonia, and Clodius Albinus in Britain, each having three legions under his command, refused to recognise the authority of Julianus.

[67] At last the Praetorians, having received assurances that they would suffer no punishment – provided they surrendered the actual murderers of Pertinax – seized the ringleaders of the conspiracy and reported what they had done to Silius Messala, the consul, by whom the Senate was summoned and informed of the proceedings.

Proclaimed emperor in 193 by his legionaries in Noricum during the political unrest that followed the death of Commodus, he secured sole rule over the empire in 197 after defeating his last rival, Clodius Albinus, at the Battle of Lugdunum.

Although coming from a humble background that was not dynastically related to the Severan dynasty, he rose through the imperial household until, under the emperor Caracalla, he was made prefect of the Praetorian Guard.

Seeing that her grandson's outrageous behaviour could mean the loss of power, Julia Maesa persuaded Elagabalus to accept his cousin Severus Alexander as Caesar (and thus the nominal emperor-to-be).

The eighteen-year-old Emperor Elagabalus and his mother were both taken from the palace, dragged through the streets, murdered and thrown in the river Tiber by the Praetorian Guard, who then proclaimed Severus Alexander as Augustus.

Many Roman legions had been defeated during a campaign against Germanic peoples raiding across the borders, while the emperor was focused primarily on the dangers from the Sassanid Persian Empire.

In particular, the right of succession had never been clearly defined in the Roman Empire, leading to continuous civil wars as competing factions in the military, Senate and other parties put forward their favoured candidate for emperor.

Although effective while he ruled, Diocletian's tetrarchic system collapsed after his abdication under the competing dynastic claims of Maxentius and Constantine, sons of Maximian and Constantius respectively.

Weakened by illness, Diocletian left the imperial office on 1 May 305, and became the first Roman emperor to voluntarily abdicate the position (John VI retired to a monastery in the 14th century).

According to Ammianus Marcellinus, during an audience with an embassy from the Quadi at Brigetio on the Danube, Valentinian suffered a burst blood vessel in his brain while angrily yelling at the people gathered,[77] resulting in his death on 17 November 375.

Some of his officers advised caution and to await the arrival of Gratian, others urged an immediate attack and eventually prevailed over Valens, who, eager to have all of the glory for himself, rushed into battle.

Maximus was a firm believer of the Nicene Creed and introduced state persecution on charges of heresy, which brought him into conflict with Pope Siricius, who argued that the augustus had no authority over church matters.

On the death of Theodosius II, the military officer Marcian was acclaimed Valentinian III's co-augustus and married the late emperor's elder sister, the augusta Pulcheria.