The Roman Empire (Latin: Imperium Romanum) was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings around the Mediterranean in Europe, Africa, and Asia. The 500-year-old Roman Republic, which preceded it, had been destabilized through a series of civil wars. Several events marked the transition from Republic to Empire, including Julius Caesar's appointment as perpetual dictator (44 BC); the Battle of Actium (2 September 31 BC); and the granting of the honorific Augustus to Octavian by the Roman Senate (16 January 27 BC).
The first two centuries of the Empire were a period of unprecedented stability and prosperity known as the Pax Romana ("Roman Peace"). It reached its greatest expanse during the reign of Trajan (98–117 AD). In the 3rd century, the Empire underwent a crisis that threatened its existence, but was reunified and stabilized under the emperors Aurelian and Diocletian. Christians rose to power in the 4th century, during which time a system of dual rule was developed in the Latin West and Greek East. After the collapse of central government in the West in the 5th century, the eastern half continued as what would later be known as the Byzantine Empire.
Because of the Empire's vast extent and long endurance, the institutions and culture of Rome had a profound and lasting influence on the development of language, religion, architecture, philosophy, law, and forms of government in the territory it governed, particularly Europe, and by means of European expansionism throughout the modern world.
Rome had begun annexing provinces in the 3rd century BC, four centuries before reaching its greatest territorial extent, and in that sense was an "empire" while still governed as a republic. Republican provinces were administered by former consuls and praetors, who had been elected to one-year terms and held imperium, "right of command". The amassing of disproportionate wealth and military power by a few men through their provincial commands was a major factor in the transition from republic to imperial autocracy. Later, the position of power held by the emperor was expressed as imperium. The Latin word is the origin of English "empire," a meaning it began to acquire only later in Rome's history.
As the first emperor, Augustus took the official position that he had saved the Republic, and carefully framed his powers within republican constitutional principles. He rejected titles that Romans associated with monarchy, and instead referred to himself as the princeps, "leading citizen". Consuls continued to be elected, tribunes of the people continued to put forth legislation, and senators still debated in the curia. It was Augustus, however, who established the precedent that the emperor controlled the final decisions, backed up by military force.
The reign of Augustus, lasting more than 40 years, was portrayed in Augustan literature and art as a new "Golden Age." Augustus laid out an enduring ideological foundation for the three centuries of the Empire known as the Principate (27 BC–284 AD), the first 200 years of which is traditionally regarded as the Pax Romana. During this period, the cohesion of the Empire was furthered by participation in civic life, economic ties, and shared cultural, legal and religious norms. Uprisings in the provinces were infrequent, but put down "mercilessly and swiftly" when they occurred, as in Britain and Gaul. The sixty years of Jewish–Roman wars in the first half of the 2nd century were exceptional in their duration and violence.
The success of Augustus in establishing principles of dynastic succession was limited by his outliving a number of talented potential heirs: the Julio-Claudian dynasty lasted for four more emperors—Tiberius, Caligula, Claudius, and Nero—before it yielded in 69 AD to the strife-torn Year of Four Emperors, from which Vespasian emerged as victor.
Vespasian became the founder of the brief Flavian dynasty, to be followed by the Nerva–Antonine dynasty which produced the "Five Good Emperors": Nerva, Trajan, Hadrian, Antoninus Pius and the philosophically inclined Marcus Aurelius. In the view of the Greek historian Dio Cassius, a contemporary observer, the accession of the emperor Commodus in 180 AD marked the descent "from a kingdom of gold to one of rust and iron"—a famous comment which has led some historians, notably Edward Gibbon, to take Commodus' reign as the beginning of the decline of the Roman Empire.
In 212, during the reign of Caracalla, Roman citizenship was granted to all freeborn inhabitants of the Empire. But 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 Roman Empire was engulfed by the Crisis of the Third Century, a period of invasions, civil strife, economic depression, and plague. In defining historical epochs, this crisis is sometimes viewed as marking the transition from Classical Antiquity to Late Antiquity. The emaciated illusion of the old Republic was sacrificed for the sake of imposing order: Diocletian (reigned 284–305) brought the Empire back from the brink, but declined the role of princeps and became the first emperor to be addressed regularly as dominus, "master" or "lord". Diocletian's reign also brought the Empire's most concerted effort against the perceived threat of Christianity, the "Great Persecution".The state of autocratic absolutism that began with Diocletius as the Dominate endured till the fall of the Western Roman Empire in 476.
The unity of the Roman Empire was from this point a fiction, as graphically revealed by Diocletian's division of authority among four "co-emperors", the Tetrarchy. Order was shaken again soon after, but was restored by Constantine, who became the first emperor to convert to Christianity, and who established Constantinople as the new capital of the eastern empire. During the decades of the Constantinian and Valentinian dynasties, the Empire was divided along an east-west axis, with dual power centres in Constantinople and Rome. The reign of Julian, who attempted to restore Classical Roman and Hellenistic religion, only briefly interrupted the succession of Christian emperors. Theodosius I, the last emperor to rule over both East and West, died in 395 AD after making Christianity the official state religion.
The Roman Empire began to disintegrate in the late 4th century as invasions overwhelmed the capacity of the Empire to govern and mount a coordinated defense. Most chronologies place the end of the Western empire in 476, when Romulus Augustulus was forced to abdicate to the Germanic warlord Odoacer. The empire in the East—known today as the Byzantine Empire, but referred to in its time as the "Roman Empire" or by various other names—ended in 1453 with the death of Constantine XI and the fall of Constantinople to the Ottoman Turks.
The Roman Empire was one of the largest in history. The Latin phrase imperium sine fine ("empire without end") expressed the ideology that neither time nor space limited the Empire. In Vergil's epic poem the Aeneid, limitless empire is said to be granted to the Romans by their supreme deity Jupiter. This claim of universal dominion was renewed and perpetuated when the Empire came under Christian rule in the 4th century.
In reality, Roman expansion was mostly accomplished under the Republic, though parts of northern Europe were conquered in the 1st century AD, when Roman control in Europe, Africa and Asia was strengthened. During the reign of Augustus, a "global map of the known world" was displayed for the first time in public at Rome, coinciding with the composition of the most comprehensive work on political geography that survives from antiquity, the Geography of the Pontic Greek writer Strabo. When Augustus died, the commemorative account of his achievements (Res Gestae) prominently featured the geographical cataloging of peoples and places within the Empire. Geography, the census, and the meticulous keeping of written records were central concerns of Roman Imperial administration.
The Empire reached its largest expanse under Trajan (reigned 98–117), encompassing an area of 5 million square kilometres that as of 2009 was divided among forty different modern countries. The traditional population estimate of 55–60 million inhabitants accounted for between one-sixth and one-fourth of the world's total population and made it the largest population of any unified political entity in the West until the mid-19th century. Recent demographic studies have argued for a population peak ranging from 70 million to more than 100 million. Each of the three largest cities in the Empire—Rome, Alexandria, and Antioch— was almost twice the size of any European city at the beginning of the 17th century.
As the historian Christopher Kelly has described it:
Then the empire stretched from Hadrian's Wall in drizzle-soaked northern England to the sun-baked banks of the Euphrates in Syria; from the great Rhine-Danube river system, which snaked across the fertile, flat lands of Europe from the Low Countries to the Black Sea, to the rich plains of the North African coast and the luxuriant gash of the Nile Valley in Egypt. The empire completely circled the Mediterranean ... referred to by its conquerors as mare nostrum—'our sea'.
Trajan's successor Hadrian adopted a policy of maintaining rather than expanding the empire. Borders (fines) were marked, and the frontiers (limes) patrolled. The most heavily fortified borders were the most unstable. Hadrian's Wall, which separated the Roman world from what was perceived as an ever-present barbarian threat, is the primary surviving monument of this effort.
The language of the Romans was Latin, which Vergil emphasises as a source of Roman unity and tradition. Until the time of Alexander Severus (reigned 222–235), the birth certificates and wills of Roman citizens had to be written in Latin. Latin was the language of the law courts in the West and of the military throughout the Empire, but was not imposed officially on peoples brought under Roman rule. St. Augustine observed that Romans preferred for Latin to be adopted per pacem societatis, through a social pact. This policy contrasts with that of Alexander the Great, who aimed to impose Greek throughout his empire as the official language. As a consequence of Alexander’s conquests, koine Greek had become the shared language around the eastern Mediterranean and into Asia Minor. The "linguistic frontier" dividing the Latin West and the Greek East passed through the Balkan peninsula, creating a bilateral monolingualism in the Roman Empire.
Romans who received an elite education studied Greek as a literary language, and most men of the governing classes could speak Greek. The Julio-Claudian emperors encouraged high standards of correct Latin (Latinitas), a linguistic movement identified in modern terms as Classical Latin, and favored Latin for conducting official business. Claudius tried to limit the use of Greek, and on occasion revoked the citizenship of those who lacked Latin, but even in the Senate he drew on his own bilingualism in communicating with Greek-speaking ambassadors. Suetonius quotes him as referring to "our two languages," and the employment of two imperial secretaries, one for Greek and one Latin, dates to his reign.
In the Eastern empire, laws and official documents were regularly translated into Greek from Latin. The everyday interpenetration of the two languages is indicated by bilingual inscriptions, which sometimes even switch back and forth between Greek and Latin. The epitaph of a Greek-speaking soldier, for instance, might be written primarily in Greek, with his rank and unit in the Roman army expressed in Latin. After all freeborn inhabitants of the empire were universally enfranchised in 212 AD, a great number of Roman citizens would have lacked Latin, though they were expected to acquire at least a token knowledge, and Latin remained a marker of "Romanness."
Among other reforms, the emperor Diocletian (reigned 284–305) sought to renew the authority of Latin, and the Greek expression ἡ κρατοῦσα διάλεκτος (hē kratousa dialektos) attests to the continuing status of Latin as "the language of power." The scholar Libanius (4th century) regarded Latin as causing a decline in the quality of Greek rhetoric. In the early 6th century, the emperor Justinian engaged in a quixotic effort to reassert the status of Latin as the language of law, even though in his time Latin no longer held any currency as a living language in the East.
Local languages 
References to interpreters indicate the continuing use of local languages other than Greek and Latin, particularly in Egypt, where Coptic predominated, and in military settings along the Rhine and Danube. Roman jurists also show a concern for local languages such as Punic, Gaulish, and Aramaic in assuring the correct understanding and application of laws and oaths. In the province of Africa, Punic was used for legends on coins during the time of Tiberius (1st century AD), and Punic inscriptions appear on public buildings into the 2nd century, some bilingual with Latin.
In the West, the importance of Latin in gaining access to the ruling power structure caused the rapid extinction of inscriptions in scripts that had been used to represent local languages on the Iberian peninsula (Roman Spain) and in Gaul. Among other aspects of a distinctive Gallo-Roman culture was the creation of Gallo-Latin text, and Mediterranean Gaul had become trilingual (Greek, Latin, Gaulish) by the mid-1st century BC. In Italy, the written use of Latin had replaced Etruscan and Oscan by the end of the 1st century AD. The dominance of Latin among the literate elite may obscure the continuity of spoken languages, since all cultures within the Roman Empire were predominantly oral.
The Babatha Archive is a suggestive example of multilingualism in the Empire. These papyri, named for a Jewish woman in the province of Arabia and dating from 93 to 132 AD, mostly employ Aramaic, the local language, written in Greek characters with Semitic and Latin influences; a petition to the Roman governor, however, was written in Greek.
Linguistic legacy 
In the Western Empire, Latin, referred to in its spoken form as Vulgar Latin, gradually replaced Celtic and Italic languages that were related to it by a shared Indo-European origin. Commonalities in syntax and vocabulary facilitated the adoption of Latin. Basque, not an Indo-European language, survived in the region of the Pyrenees. After the decentralization of political power in late antiquity, Latin developed locally into branches that became the Romance languages, such as Spanish, Portuguese, French, Italian and Romanian. As an international language of learning and literature, Latin itself continued as an active medium of expression for diplomacy and for intellectual developments identified with Renaissance humanism up to the 17th century, and for law and the Roman Catholic Church to the present.
Although Greek continued as the language of the Byzantine Empire, linguistic distribution in the East was more complex. A Greek-speaking majority lived in the Greek peninsula and islands, western Anatolia, major cities, and some coastal areas. Like Greek and Latin, the Thracian language was of Indo-European origin, as were several now-extinct languages in Anatolia such as Galatian, Phrygian, Pisidian, and Cappadocian, attested by Imperial-era inscriptions. Various Afroasiatic languages—primarily Coptic in Egypt, and Aramaic in Syria and Mesopotamia—were never replaced by Greek. Palmyrene soldiers even used their dialect of Aramaic for inscriptions, in a striking exception to the rule that Latin was the language of the military. The international use of Greek, however, was one factor enabling the spread of Christianity, as indicated for example by the use of Greek for the Epistles of Paul.
The Roman Empire was remarkably multicultural, with "a rather astonishing cohesive capacity" to create a sense of shared identity while encompassing diverse peoples within its political system over a long span of time. The Roman attention to creating public monuments and communal spaces open to all—such as forums, amphitheaters, racetracks and baths—helped foster a sense of "Romanness".
Roman society had multiple, overlapping social hierarchies that modern concepts of "class" in English may not represent accurately. The two decades of civil war from which Augustus rose to sole power left traditional society in Rome in a state of confusion and upheaval, but did not effect an immediate redistribution of wealth and social power. From the perspective of the lower classes, a peak was merely added to the social pyramid. Personal relationships—patronage, friendship (amicitia), family, marriage—continued to influence the workings of politics and government, as they had in the Republic. By the time of Nero, however, it was not unusual to find a former slave who was richer than a freeborn citizen, or an equestrian who exercised greater power than a senator.
The blurring or diffusion of the Republic's more rigid hierarchies led to increased social mobility under the Empire, both upward and downward, to an extent that exceeded that of all other well-documented ancient societies. Women, freedmen, and slaves had opportunities to profit and exercise influence in ways previously less available to them. Social life in the Empire, particularly for those whose personal resources were limited, was further fostered by a proliferation of voluntary associations and confraternities (collegia and sodalitates) formed for various purposes: professional and trade guilds, veterans' groups, religious sodalities, drinking and dining clubs, performing arts troupes, and burial societies.
Legal status 
According to the jurist Gaius, the essential distinction in the Roman "law of persons" was that all human beings were either free (liberi) or slaves (servi). The legal status of free persons might be further defined by their citizenship. In the early Empire, only a relatively limited number of men held full rights of Roman citizenship that allowed them to vote, run for office, and enter state priesthoods. Most citizens held limited rights (such as the ius Latinum, "Latin right"), but were entitled to legal protections and privileges not enjoyed by those who lacked citizenship. Free people not considered citizens, but living within the Roman world, held status as peregrini, non-Romans. In 212 AD, by means of the edict known as the Constitutio Antoniniana, the emperor Caracalla extended citizenship to all freeborn inhabitants of the empire. This legal egalitarianism would have required a far-reaching revision of existing laws that had distinguished between citizens and non-citizens.
Women as legal entities 
Freeborn Roman women were considered citizens throughout the Republic and Empire, but did not vote, hold political office, or serve in the military. A mother's citizen status determined that of her children, as indicated by the phrase ex duobus civibus Romanis natos ("children born of two Roman citizens"). A Roman woman kept her own family name (nomen) for life. Children most often took the father's name, but in the Imperial period sometimes made their mother's name part of theirs, or even used it instead.
The archaic form of manus marriage in which the woman had been subject to her husband's authority was largely abandoned by the Imperial era, and a married woman retained ownership of any property she brought into the marriage. Technically she remained under her father's legal authority, even though she moved into her husband's home, but when her father died she became legally emancipated. This arrangement was one of the factors in the degree of independence Roman women enjoyed relative to those of many other ancient cultures and up to the modern period: although she had to answer to her father in legal matters, she was free of his direct scrutiny in her daily life, and her husband had no legal power over her. Although it was a point of pride to be a "one-man woman" (univira) who had married only once, there was little stigma attached to divorce, nor to speedy remarriage after the loss of a husband through death or divorce.
Girls had equal inheritance rights with boys if their father died without leaving a will. A Roman mother's right to own property and to dispose of it as she saw fit, including setting the terms of her own will, gave her enormous influence over her sons even when they were adults.
As part of the Augustan programme to restore traditional morality and social order, moral legislation attempted to regulate the conduct of women as a means of promoting "family values". Adultery, which had been a private family matter under the Republic, was criminalized, and defined broadly as an illicit sex act (stuprum) that occurred between a male citizen and a married woman, or between a married woman and any man other than her husband. Childbearing was encouraged by the state: a woman who had given birth to three children was granted symbolic honors and greater legal freedom (the ius trium liberorum).
Because of their legal status as citizens and the degree to which they could become emancipated, women could own property, enter contracts, and engage in business, including shipping, manufacturing, and lending money. Inscriptions throughout the Empire honor women as benefactors in funding public works, an indication they could acquire and dispose of considerable fortunes; for instance, the Arch of the Sergii was funded by Salvia Postuma, a female member of the family honored, and the largest building in the forum at Pompeii was funded by Eumachia, a priestess of Venus.
Slaves and the law 
At the time of Augustus, as many as 35 percent of the people in Italy were slaves, making Rome one of five historical "slave societies" in which slaves constituted at least a fifth of the population and played a major role in the economy. Slavery was a complex institution that supported traditional Roman social structures as well as contributing economic utility. In urban settings, slaves might be professionals such as teachers, physicians, chefs, and accountants, in addition to the majority of slaves who provided trained or unskilled labor in households or workplaces. Agriculture and industry, such as milling and mining, relied on the exploitation of slaves. Outside Italy, slaves made up on average an estimated 10 to 20 percent of the population, sparse in Roman Egypt but more concentrated in some Greek areas. Expanding Roman ownership of arable land and industries would have affected preexisting practices of slavery in the provinces.
Laws pertaining to slavery were "extremely intricate". Under Roman law, slaves were considered property and had no legal personhood. They could be subjected to forms of corporal punishment not normally exercised on citizens, sexual exploitation, torture, and summary execution. A slave could not as a matter of law be raped, since rape could be committed only against people who were free; a slave's rapist had to be prosecuted by the owner for property damage under the Aquilian Law. Slaves had no right to the form of legal marriage called conubium, but their unions were sometimes recognized, and if both were freed they could marry. Following the Servile Wars of the Republic, legislation under Augustus and his successors shows a driving concern for controlling the threat of rebellions through limiting the size of work groups, and for hunting down fugitive slaves.
Technically, a slave could not own property, but a slave who conducted business might be given access to an individual account or fund (peculium) that he could use as if it were his own. The terms of this account varied depending on the degree of trust and cooperation between owner and slave: a slave with an aptitude for business could be given considerable leeway to generate profit, and might be allowed to bequeath the peculium he managed to other slaves of his household. Within a household or workplace, a hierarchy of slaves might exist, with one slave in effect acting as the master of other slaves.
Over time slaves gained increased legal protection, including the right to file complaints against their masters. A bill of sale might contain a clause stipulating that the slave could not be employed for prostitution, as prostitutes in ancient Rome were often slaves. The burgeoning trade in eunuch slaves in the late 1st century AD prompted legislation that prohibited the castration of a slave against his will "for lust or gain."
Roman slavery was not based on "race" in the modern sense. During the period of Republican expansionism when slavery had become pervasive, war captives were a main source of slaves. The range of ethnicities among slaves to some extent reflected that of the armies Rome defeated in war, and the conquest of Greece brought a number of highly skilled and educated slaves into Rome. Slaves were also traded in markets, and sometimes sold by pirates. Infant abandonment and self-enslavement among the poor were other sources. Vernae, by contrast, were "homegrown" slaves born to female slaves within the urban household or on a country estate or farm. Although they had no special legal status, an owner who mistreated or failed to care for his vernae faced social disapproval, as they were considered part of his familia, the family household, and in some cases might actually be the children of free males in the family.
Highly skilled or educated slaves might accumulate a large enough peculium to justify their freedom, or be manumitted for services rendered. Manumission had become frequent enough that in 2 BC a law (Lex Fufia Caninia) limited the number of slaves an owner was allowed to free in his will.
Rome differed from Greek city-states in allowing freed slaves to become citizens. After manumission, a slave who had belonged to a Roman citizen enjoyed not only passive freedom from ownership, but active political freedom (libertas), including the right to vote. A slave who had acquired libertas was a libertus ("freed person," feminine liberta) in relation to his former master, who then became his patron (patronus): the two parties continued to have customary and legal obligations to each other. As a social class generally, freed slaves were libertini, though later writers used the terms libertus and libertinus interchangeably.
A libertinus was not entitled to hold public office or the highest state priesthoods, but he could play a priestly role in the cult of the emperor. He could not marry a woman from a family of senatorial rank, nor achieve legitimate senatorial rank himself, but during the early Empire, freedmen held key positions in the government bureaucracy, so much so that Hadrian limited their participation by law. Any future children of a freedman would be born free, with full rights of citizenship.
The rise of successful freedmen—through either political influence in imperial service, or wealth—is a characteristic of early Imperial society. The prosperity of a high-achieving group of freedmen is attested by inscriptions throughout the Empire, and by their ownership of some of the most lavish houses at Pompeii, such as the House of the Vettii. The excesses of nouveau riche freedmen were satirized in the character of Trimalchio in the Satyricon by Petronius, who wrote in the time of Nero. Such individuals, while exceptional, are indicative of the upward social mobility possible in the Empire.
Census rank 
The Latin word ordo (plural ordines) refers to a social distinction that is translated variously into English as "class, order, rank," none of which is exact. One purpose of the Roman census was to determine the ordo to which an individual belonged. The two highest ordines in Rome were the senatorial and equestrian. Outside Rome, the decurions, also known as curiales (Greek bouleutai), were the top governing ordo of an individual city.
"Senator" was not itself an elected office in ancient Rome; an individual gained admission to the Senate after he had been elected to and served at least one term as an executive magistrate. A senator also had to meet a minimum property requirement of 1 million sestertii, as determined by the census. Nero made large gifts of money to a number of senators from old families who had become too impoverished to qualify. Not all men who qualified for the ordo senatorius chose to take a Senate seat, which required legal domicile at Rome. Emperors often filled vacancies in the 600-member body by appointment. A senator's son belonged to the ordo senatorius, but he had to qualify on his own merits for admission to the Senate itself. A senator could be removed for violating moral standards: he was prohibited, for instance, from marrying a freedwoman or fighting in the arena.
In the time of Nero, senators were still primarily from Rome and other parts of Italy, with some from the Iberian peninsula and southern France; men from the Greek-speaking provinces of the East began to be added under Vespasian. The first senator from the most eastern province, Cappadocia, was admitted under Marcus Aurelius. During the 3rd century, domicile at Rome became impractical, and inscriptions attest to senators who were active in politics and munificence in their homeland (patria).
Senators had an aura of prestige and were the traditional governing class who rose through the cursus honorum, the political career track, but equestrians of the Empire often possessed greater wealth and political power. Membership in the equestrian order was based on property; in Rome's early days, equites or knights had been distinguished by their ability to serve as mounted warriors (the "public horse"), but cavalry service was a separate function in the Empire. A census valuation of 400,000 sesterces and three generations of free birth qualified a man as an equestrian. The census of 28 BC uncovered large numbers of men who qualified, and in 14 AD, a thousand equestrians were registered at Cadiz and Padua alone. Equestrians rose through a military career track (tres militiae) to become highly placed prefects and procurators within the Imperial administration.
The rise of provincial men to the senatorial and equestrian orders is an aspect of social mobility in the first three centuries of the Empire. Roman aristocracy was based on competition, and unlike later European nobility, a Roman family could not maintain its position merely through hereditary succession or having title to lands. Admission to the higher ordines brought distinction and privileges, but also a number of responsibilities. In antiquity, a city depended on its leading citizens to fund public works, events, and services (munera), rather than on tax revenues, which primarily supported the military. Maintaining one's rank required massive personal expenditures. Decurions were so vital for the functioning of cities that in the later Empire, as the ranks of the town councils became depleted, those who had risen to the Senate were encouraged by the central government to give up their seats and return to their hometowns, in an effort to sustain civic life.
In the later Empire, the dignitas ("worth, esteem") that attended on senatorial or equestrian rank was refined further with titles such as vir illustris, "illustrious man". The appellation clarissimus (Greek lamprotatos) was used to designate the dignitas of certain senators and their immediate family, including women. "Grades" of equestrian status proliferated. Those in Imperial service were ranked by pay grade (sexagenarius, 60,000 sesterces per annum; centenarius, 100,000; ducenarius, 200,000). The title eminentissimus, "most eminent" (Greek exochôtatos) was reserved for equestrians who had been Praetorian prefects. The higher equestrian officials in general were perfectissimi, "most distinguished" (Greek diasêmotatoi), the lower merely egregii, "outstanding" (Greek kratistos).
Unequal justice 
As the republican principle of citizens' equality under the law faded, the symbolic and social privileges of the upper classes led to an informal division of Roman society into those who had acquired greater honors (honestiores) and those who were humbler folk (humiliores). In general, honestiores were the members of the three higher "orders," along with certain military officers. The granting of universal citizenship in 212 seems to have increased the competitive urge among the upper classes to have their superiority over other citizens affirmed, particularly within the justice system. Sentencing depended on the judgment of the presiding official as to the relative "worth" (dignitas) of the defendant: an honestior could pay a fine when convicted of a crime for which an humilior might receive a scourging.
Execution, which had been an infrequent legal penalty for free men under the Republic even in a capital case, could be quick and relatively painless for the Imperial citizen considered "more honorable," while those deemed inferior might suffer the kinds of torture and prolonged death previously reserved for slaves, such as crucifixion and condemnation to the beasts as a spectacle in the arena. In the early Empire, those who converted to Christianity could lose their standing as honestiores, especially if they declined to fulfill the religious aspects of their civic responsibilities, and thus became subject to punishments that created the conditions of martyrdom.
Government and military 
The three major elements of the Imperial Roman state were the central government, the military, and provincial government. The military established control of a territory through war, but after a city or people was brought under treaty, the military mission turned to policing: protecting Roman citizens (after 212 AD, all freeborn inhabitants of the Empire), the agricultural fields that fed them, and religious sites. Without modern instruments of either mass communication or mass destruction, the Romans lacked sufficient manpower or resources to impose their rule through force alone. Cooperation with local power elites was necessary to maintain order, collect information, and extract revenue. The Romans often exploited internal political divisions by supporting one faction over another: in the view of Plutarch, "it was discord between factions within cities that led to the loss of self-governance".
Communities with demonstrated loyalty to Rome retained their own laws, could collect their own taxes locally, and in exceptional cases were exempt from Roman taxation. Legal privileges and relative independence were an incentive to remain in good standing with Rome. Roman government was thus limited, but efficient in its use of the resources available to it.
Central government 
The dominance of the emperor was based on the consolidation of certain powers from several republican offices, including the inviolability of the tribunes of the people and the authority of the censors to manipulate the hierarchy of Roman society. The emperor also made himself the central religious authority as Pontifex Maximus, and centralized the right to declare war, ratify treaties, and negotiate with foreign leaders. While these functions were clearly defined during the Principate, the emperor's powers over time became less constitutional and more monarchical, culminating in the Dominate.
The emperor was the ultimate authority in policy- and decision-making, but in the early Principate he was expected to be accessible to individuals from all walks of life, and to deal personally with official business and petitions. A bureaucracy formed around him only gradually. The Julio-Claudian emperors relied on an informal body of advisors that included not only senators and equestrians, but trusted slaves and freedmen. After Nero, the unofficial influence of the latter was regarded with suspicion, and the emperor's council (consilium) became subject to official appointment for the sake of greater transparency. Though the senate took a lead in policy discussions until the end of the Antonine dynasty, equestrians played an increasingly important role in the consilium. The women of the emperor's family often intervened directly in his decisions. Plotina exercised influence on both her husband Trajan and his successor Hadrian. Her influence was advertised by having her letters on official matters published, as a sign that the emperor was reasonable in his exercise of authority and listened to his people.
Access to the emperor by others might be gained at the daily reception (salutatio), a development of the traditional homage a client paid to his patron; public banquets hosted at the palace; and religious ceremonies. The common people who lacked this access could manifest their general approval or displeasure as a group at the games held in large venues. By the 4th century, as urban centers decayed, the Christian emperors became remote figureheads who issued general rulings, no longer responding to individual petitions.
Although the senate could do little short of assassination and open rebellion to contravene the will of the emperor, it survived the Augustan restoration and the turbulent Year of Four Emperors to retain its symbolic political centrality during the Principate. The senate legitimated the emperor's rule, and the emperor needed the experience of senators as legates (legati) to serve as generals, diplomats, and administrators. A successful career required competence as an administrator and remaining in favor with the emperor, or over time perhaps multiple emperors.
The practical source of an emperor's power and authority was the military. The legionaries were paid by the Imperial treasury, and swore an annual military oath of loyalty to the emperor (sacramentum). The death of an emperor led to a crucial period of uncertainty and crisis. Most emperors indicated their choice of successor, usually a close family member or adopted heir. The new emperor had to seek a swift acknowledgement of his status and authority in order to stabilize the political landscape. No emperor could hope to survive, much less to reign, without the allegiance and loyalty of the Praetorian Guard and of the legions. To secure their loyalty, several emperors paid the donativum, a monetary reward. In theory, the Senate was entitled to choose the new emperor, but did so mindful of acclamation by the army or Praetorians.
The soldiers of the Imperial Roman army were professionals who volunteered for a 25-year tour of duty. The transition to a professional military had begun during the late Republic, and was one of the many profound shifts away from republicanism, under which an army of conscripts had exercised their responsibilities as citizens in defending the homeland in a campaign against a specific threat. For Imperial Rome, the military was a full-time career in itself.
- the garrison at Rome, which includes both the Praetorians and the vigiles who functioned as police and firefighters;
- the provincial army, comprising the Roman legions and the auxiliaries provided by the provinces (auxilia);
- the navy.
The pervasiveness of military garrisons throughout the Empire was a major influence in the process of cultural exchange and assimilation known as "Romanization," particularly in regard to politics, the economy, and religion. Knowledge of the Roman military comes from a wide range of sources: Greek and Roman literary texts; coins with military themes; papyri preserving military documents; monuments such as Trajan's Column and triumphal arches, which feature artistic depictions of both fighting men and military machines; the archaeology of military burials, battle sites, and camps; and inscriptions, including military diplomas, epitaphs, and dedications.
Through his military reforms, which included consolidating or disbanding units of questionable loyalty, Augustus changed and regularized the legion, down to the hobnail pattern on the soles of army boots. A legion was organized into ten cohorts, each of which comprised six centuries, with a century further made up of ten squads (contubernia); the exact size of the Imperial legion, which is most likely to have been determined by logistics, has been estimated to range from 4,800 to 5,280.
In AD 9, Germanic tribes wiped out three full legions in the Battle of the Teutoburg Forest. This disastrous event reduced the number of the legions to 25. The total of the legions would later be increased again and for the next 300 years always be a little above or below 30. The army had about 300,000 soldiers in the 1st century, and under 400,000 in the 2nd, "significantly smaller" than the collective armed forces of the territories it conquered. No more than 2 percent of adult males living in the Empire served in the Imperial army.
Augustus also created the Praetorian Guard: nine cohorts, ostensibly to maintain the public peace, which were garrisoned in Italy. Better paid than the legionaries, the Praetorians served only sixteen years.
The auxilia were recruited from among the non-citizens. Organized in smaller units of roughly cohort strength, they were paid less than the legionaries, and after 25 years of service were rewarded with Roman citizenship, also extended to their sons. According to Tacitus there were roughly as many auxiliaries as there were legionaries. The auxilia thus amounted to around 125,000 men, implying approximately 250 auxiliary regiments. The Roman cavalry of the earliest Empire were primarily from Celtic and Germanic areas, or Roman Spain. Several aspects of training and equipment, such as the four-horned saddle, derived from the Celts, as noted by Arrian and indicated by archaeology.
The Roman navy (Latin: classis, "fleet") not only aided in the supply and transport of the legions, but also helped in the protection of the frontiers along the rivers Rhine and Danube. Another of its duties was the protection of the crucial maritime trade routes against the threat of pirates. It patrolled the whole of the Mediterranean, parts of the North Atlantic coasts, and the Black Sea. Nevertheless the army was considered the senior and more prestigious branch.