The Battle of Actium resulted in the defeat and subsequent suicides of Mark Antony and Cleopatra. Octavian also had executed Cleopatra's young son and co-ruler, Caesarion. Caesarion may have been the (only) son of Julius Caesar. Therefore, by killing Caesarion, Octavian removed any possibility of a male rival emerging with closer blood ties to Julius Caesar. Octavian, now sole ruler of Rome, began a fullscale reformation of military, fiscal and political matters. These reforms were intended to stabilize and to pacify the Roman world and would also cement the acceptance of the new regime.

Upon Octavian's accedance as ruler of the Roman world, the Roman Senate gave Octavian the name "Augustus." He had adopted the title "imperator," commander-in-chief, as his first name already earlier. It was a term that dated back to the days of the Republic and later evolved into "emperor."

As adopted heir of Caesar, Augustus preferred to be called by this name. This usage was different from later usages of the word. "Caesar" was a component of his family name. Julio-Claudian rule lasted for almost a century (from Julius Caesar in the mid 1st century BC to the emperor Nero in the mid 1st century AD). By the time of the Flavian Dynasty, and the reign of Vespasian, and that of his two sons, Titus and Domitian, the term "Caesar" evolved, almost de facto, from a family name into a formal title. Derivatives of this title (such as Czar) last to this day.

The Roman legions, which had reached an unprecedented number (around sixty) because of the civil wars, were reduced to twenty-eight. Several legions, particularly those with members of doubtful loyalties, were simply disbanded. Other legions were amalgamated, a fact hinted by the title Gemina ("Twin"). Augustus also created nine special cohorts, ostensibly to maintain the peace in Italy, keeping at least three of them stationed at Rome. These cohorts became known as the Praetorian Guard.

Octavian realized that autocracy and kingship were things that Romans had not experienced for centuries, and were wary of. Octavian wanted to retain the illusion of the constitutional republic, and thus did not want to be viewed as a tyrant. Even Rome's past dictators, such as the brutal Lucius Cornelius Sulla, had only ruled Rome for short spans of time, never more than a year or two (with the exception of Julius Caesar). In 27 BC, Octavian officially tried to relinquish all his extraordinary powers back to the Roman Senate. In a carefully staged way the senators, who by this time were mostly his partisans, refused and begged him to continue for the sake of the republic and the people of Rome. Reportedly, the suggestion of Octavian's stepping down as consul led to rioting amongst the Plebeians in Rome. A compromise was reached between the Senate and Octavian, known as the First Settlement. This agreement gave Augustus legitimacy as an autocrat of the people, and ensured that he would not be viewed as a tyrant.

Octavian split with the Senate the governorships of the provinces. The unruly provinces at the borders, where the vast majority of the legions were stationed, were administrated by imperial legates, chosen by the emperor himself. These provinces were classified as imperial provinces. The governors of the peaceful senatorial provinces were chosen by the Senate. These provinces were usually peaceful and only a single legion was stationed in the senatorial province of Africa.



Before the Senate controlled the treasury, Augustus had mandated that the taxes of the Imperial provinces were destined to the Fiscus, which was administrated by persons chosen and answerable only to Augustus. The revenue of the senatorial provinces continued to be sent to the Aerarium, under the supervision of the Senate. This effectively made Augustus richer than the Senate, and more than able to pay the salarium ("salary") of the legionaries, ensuring their continued loyalty. This was ensured by the Imperial province of Aegyptus. This province was incredibly wealthy and also the most important grain supplier for the whole empire. Senators were forbidden to even visit this province, as it was largely considered the personal fiefdom of the emperor himself.

Augustus renounced his consulship in 23 BC, but retained his consular imperium, leading to a second compromise between Augustus and the Senate known as the Second Settlement. Augustus was granted the authority of a tribune (tribunicia potestas), though not the title, which allowed him to convene the Senate and people at will and lay business before it, veto the actions of either the Assembly or the Senate, preside over elections, and gave him the right to speak first at any meeting. Also included in Augustus' tribunician authority were powers usually reserved for the Roman censor; these included the right to supervise public morals and scrutinize laws to ensure they were in the public interest, as well as the ability to hold a census and determine the membership of the Senate. No tribune of Rome ever had these powers, and there was no precedent within the Roman system for consolidating the powers of the tribune and the censor into a single position, nor was Augustus ever elected to the office of Censor. Whether censorial powers were granted to Augustus as part of his tribunician authority, or he simply assumed these responsibilities, is still a matter of debate.

In addition to tribunician authority, Augustus was granted sole imperium within the city of Rome itself; all armed forces in the city, formerly under the control of the praefects, were now under the sole authority of Augustus. Additionally, Augustus was granted imperium proconsulare maius ("power over all proconsuls"), the right to interfere in any province and override the decisions of any governor. With maius imperium, Augustus was the only individual able to grant a triumph to a successful general as he was ostensibly the leader of the entire Roman army.



All of these reforms were highly abnormal in the eyes of Roman republican tradition, but the Senate was no longer composed of republican patricians who had had the courage to murder Caesar. Most of these senators had died in the Civil Wars, and the leaders of the conservative Republicans in the senate, such as Cato and Cicero, had long since died. Octavian had purged the Senate of any remaining suspect elements and planted the body with his own partisans. How free a hand the Senate had in all these transactions, and what backroom deals were made, remains unknown.

Attempting to secure the borders of the empire upon the rivers Danube and Elbe, Octavian ordered the invasions of Illyria, Moesia, and Pannonia (south of the Danube), and Germania (west of the Elbe). At first everything went as planned, but then disaster struck. The Illyrian tribes revolted and had to be crushed, and three full legions under the command of Publius Quinctilius Varus were ambushed and destroyed at the Battle of the Teutoburg Forest in AD 9 by German barbarians under the leadership of Arminius. Being cautious, Augustus secured all territories west of Rhine and contented himself with retaliatory raids. The rivers Rhine and Danube became the permanent borders of the Roman empire in the North.

In peacetime, it was relatively easy to rule the empire from its capital city, Rome. An eventual rebellion was expected and would happen from time to time: a general or a governor would gain the loyalty of his officers through a mixture of personal charisma, promises and simple bribes. A conquered tribe would rebel, or a conquered city would revolt. This would be a bad, but not a catastrophic event. The Roman legions were spread around the borders and the rebel leader would in normal circumstances have only one or two legions under his command. Loyal legions would be detached from other points of the empire and would eventually drown the rebellion in blood. This happened even more easily in cases of a small local native uprising as the rebels would normally have no great military experience. Unless the emperor was weak, incompetent, hated, and/or universally despised, these rebellions would be a local and isolated event.

During real wartime however, which could develop from a rebellion or an uprising, like the massive Jewish rebellion, this was totally and dangerously different. In a full-blown military campaign, the legions under the command of the generals like Vespasian were of a much greater number. Therefore a paranoid or wise emperor would hold some members of the general's family as hostages, to make certain of the latter's loyalty. In effect, Nero held Domitian and Quintus Petillius Cerialis the governor of Ostia, who were respectively the younger son and the brother-in-law of Vespasian. In normal circumstances this would be quite enough. In fact, the rule of Nero ended with the revolt of the Praetorian Guard who had been bribed in the name of Galba. It became all too obvious that the Praetorian Guard was a sword of Damocles, whose loyalty was all too often bought and who became increasingly greedy. Following their example the legions at the borders would also increasingly participate in the civil wars. This was a dangerous development as this would weaken the whole Roman Army.

The main enemy in the West were, arguably, the "barbarian tribes" behind the Rhine and the Danube. Augustus had tried to conquer them, but ultimately failed and these "barbarians" were greatly feared. But by and large they were left in peace, in order to fight amongst themselves, and were simply too divided to pose a serious threat.



In the East lay the empire of Parthia (Persia). Crassus, a member of the First Triumvirate during the late republic, attempted an invasion in 53 BC, but was defeated by Persian forces led by Surena in the Battle of Carrhae. It was simply too far away to be conquered and then to be held. Any Parthian invasion was confronted and usually defeated, but the threat itself was ultimately impossible to destroy. Parthia would eventually become Rome's greatest rival and foremost enemy.

In the case of a Roman civil war these two enemies would seize the opportunity to invade Roman territory in order to raid and plunder. The two respective military frontiers became a matter of major political importance because of the high number of legions stationed there. All too often the local generals would rebel, starting a new civil war. To control the western border from Rome was easy, as it was relatively close. To control both frontiers, at the same time, during wartime, was somewhat more difficult. If the emperor was near the border in the East, chances were high that an ambitious general would rebel in the West and vice-versa. It was no longer enough to be a good administrator; emperors were increasingly near the troops in order to control them and no single Emperor could be at the two frontiers at the same time. This problem would plague the ruling emperors time and time again and many future emperors would follow this path to power.

The Severan dynasty includes the increasingly troubled reigns of Septimius Severus (193–211), Caracalla (211–217), Macrinus (217–218), Elagabalus (218–222), and Alexander Severus (222–235). The founder of the dynasty, Lucius Septimius Severus, belonged to a leading native family of Leptis Magna in Africa who allied himself with a prominent Syrian family by his marriage to Julia Domna. Their provincial background and cosmopolitan alliance, eventually giving rise to imperial rulers of Syrian background, Elagabalus and Alexander Severus, testifies to the broad political franchise and economic development of the Roman empire that had been achieved under the Antonines. A generally successful ruler, Septimius Severus cultivated the army's support with substantial remuneration in return for total loyalty to the emperor and substituted equestrian officers for senators in key administrative positions. In this way, he successfully broadened the power base of the imperial administration throughout the empire, also by abolishing the regular standing jury courts of Republican times.

Septimius Severus's son, Marcus Aurelius Antoninus — nicknamed Caracalla — removed all legal and political distinction between Italians and provincials, enacting the Constitutio Antoniniana in 212 which extended full Roman citizenship to all free inhabitants of the empire. Caracalla was also responsible for erecting the famous Baths of Caracalla in Rome, their design serving as an architectural model for many subsequent monumental public buildings. Increasingly unstable and autocratic, Caracalla was assassinated by the praetorian prefect Macrinus in 217, who succeeded him briefly as the first emperor not of senatorial rank. The imperial court, however, was dominated by formidable women who arranged the succession of Elagabalus in 218, and Alexander Severus, the last of the dynasty, in 222. In the last phase of the Severan principate, the power of the Senate was somewhat revived and a number of fiscal reforms were enacted. Despite early successes against the Sassanian Empire in the East, Alexander Severus's increasing inability to control the army led eventually to its mutiny and his assassination in 235. The death of Alexander Severus ushered in a subsequent period of soldier-emperors and almost a half-century of civil war and strife.

The transition from a single united empire to the later divided Western and Eastern empires was a gradual transformation. In July 285, Diocletian defeated rival Emperor Carinus and briefly became sole emperor of the Roman Empire.

Diocletian saw that the vast Roman Empire was ungovernable by a single emperor in the face of internal pressures and military threats on two fronts. He therefore split the Empire in half along a north-west axis just east of Italy, and created two equal Emperors to rule under the title of Augustus. Diocletian was Augustus of the eastern half, and gave his long-time friend Maximian the title of Augustus in the western half. In doing so, Diocletian created what would become the Western Roman Empire and the Eastern Roman Empire. The western empire would collapse less than 200 years later, and the eastern empire would become the Byzantine Empire, centered in the Greek city of Byzantium, which would later be renamed Constantinople by the emperor Constantine the Great, and would survive another one thousand years.

In 293 authority was further divided, as each Augustus took a junior Emperor called Caesar to aid him in administrative matters, and to provide a line of succession; Galerius became Caesar under Diocletian and Constantius Chlorus Caesar under Maximian. This constituted what is called the Tetrarchy (in Greek: the leadership of four) by modern scholars. After Rome had been plagued by bloody disputes about the supreme authority, this finally formalized a peaceful succession of the emperor: in each half the Caesar rose up to replace the Augustus and proclaimed a new Caesar. On May 1 305, Diocletian and Maximian abdicated in favor of their Caesars. Galerius named the two new Caesars: his nephew Maximinus for himself and Flavius Valerius Severus for Constantius. The arrangement worked well under Diocletian and Maximian and shortly thereafter. The internal tensions within the Roman government were less acute than they had been. In The Decline and Fall of the Roman Empire, Edward Gibbon notes that this arrangement worked well because of the affinity the four rulers had for each other. Gibbon says that this arrangement has been compared to a "chorus of music." With the withdrawal of Diocletian and Maximian, this harmony disappeared.

The Tetrarchy would effectively collapse with the death of Constantius Chlorus on July 25 306. Constantius's troops in Eboracum immediately proclaimed his son Constantine an Augustus. In August 306, Galerius promoted Severus to the position of Augustus. A revolt in Rome supported another claimant to the same title: Maxentius, son of Maximian, who was proclaimed Augustus on October 28, 306. His election was supported by the Praetorian Guard. This left the Empire with five rulers: four Augusti (Galerius, Constantine, Severus and Maxentius) and a Caesar (Maximinus).

The year 307 saw the return of Maximian to the role of Augustus alongside his son Maxentius, creating a total of six rulers of the Empire. Galerius and Severus campaigned against them in Italy. Severus was killed under command of Maxentius on September 16 307. The two Augusti of Italy also managed to ally themselves with Constantine by having Constantine marry Fausta, the daughter of Maximian and sister of Maxentius. At the end of 307, the Empire had four Augusti (Maximian, Galerius, Constantine and Maxentius) and a sole Caesar.