20 Roman Emperors Who Defined Ancient Rome

The Roman emperors are some of the most fascinating historical figures. They held multiple roles: army commanders, religious heads, lawmakers, and builders. These emperors steered the Roman Empire’s destiny, fortifying its frontiers, establishing and upholding numerous urban centers, assigning officials, instituting reforms, conducting sacred ceremonies, and leading troops against foreign and domestic foes. Figures like Augustus, the first emperor, ushered in a golden age. In contrast, rulers such as soldier-usurper Maximinus Thrax tipped the Empire into turmoil. Some emperors ruled for mere months, while others sat on the throne for decades, founding powerful dynasties. Yet, all of them, whether benevolent leaders, tyrants, or ruthless military men, tried their best to govern the vast Empire. Changing the state’s structure and direction, reshaping history itself. Here are the most important Roman Emperors. 

Julius Caesar was the first Roman leader to (briefly) hold near-absolute power, but it was his adopted son, Octavian, who became the first Roman emperor, Augustus. Throughout his four-decades-long reign (27 BCE –14 CE), Augustus laid the foundation for the Roman Empire. He inaugurated the first imperial dynasty, heralding an era of peace and prosperity known as the Pax Romana.

Wielding his extensive powers, Augustus enacted a series of reforms, expanded Roman territories, and fortified the borders. As a patron of the arts and literature, he commissioned numerous public buildings, monuments, and sculptures across Rome and its provinces. Augustus also expanded Rome’s horizons to the East by annexing Egypt and initiating the lucrative Indian Ocean trade, which inundated Rome with exotic luxuries.

While having absolute power and sole command over the Roman army, Augustus trod carefully, mindful of Caesar’s fate. He cleverly presented himself as the “princeps” or the “first citizen,” heralding the era of the early Empire or the Principate. By founding the first imperial dynasty, Augustus secured a peaceful power transfer, ensuring the Roman Empire could flourish for centuries.

2. Tiberius: The Reluctant Leader

Tiberius ascended the throne as the second Roman emperor. Leading Rome for over two decades 14 CE – 37 CE), Tiberius did his best to uphold Augustus’ legacy. As a seasoned general, Tiberius was instrumental in establishing Rome’s northern borders. Despite his initial reluctance to take the purple, possibly feeling unworthy to be Augustus’ heir, Tiberius emerged as a meticulous administrator, enhancing the Empire’s strength and prosperity.

Nevertheless, Tiberius harbored a deep mistrust for the Senate, fearing conspiracies. His discomfort grew into paranoia, prompting his retreat to Capri in 26 CE. This seclusion fueled rumors of debauchery and tyranny, though these accounts are often considered exaggerated by contemporary scholars. Tiberius’ absence from Rome indeed left a void, leading to political machinations and corruption.

Tiberius left Rome in the hands of powerful Praetorian prefect Sejanus, a grey eminence behind the throne. While Tiberius was blamed for the sudden death of his heir and adopted son, the popular general Germanicus, the main culprit was probably Sejanus, who harbored imperial ambitions. However, Sejanus, too, fell as a plot victim, and following Tiberius’ death, the throne came to Germanicus’ only surviving son, Caligula.

Emperor Caligula’s four-year reign (37 – 41 CE) is undoubtedly one of the most controversial periods in Roman history. When he ascended the throne, Gaius Caesar was everything his predecessor was not: young, charismatic, and widely popular. Initially, he maintained good relations with the Senate, reduced taxes, held public games, and pardoned exiles. However, after a brief illness, Caligula’s reign took a darker turn. Caligula became a paranoid, vengeful ruler who engaged in open war with the Senate. 

While ancient sources depict a reign characterized by unbridled cruelty and madness, contemporary scholars question the reliability of those lurid tales, suggesting that some of those accounts may have been exaggerated or fabricated by his political enemies. Stories like declaring emperor’s favorite horse Incitatus a consul might be a symbolic mockery of the senators rather than a literal appointment, and Caligula’s war against the sea might have merely been a prelude to an abortive conquest of Britain. 

However, Caligula’s obsession with the Hellenistic East culminated in his self-proclamation as a deity and a plan to relocate the capital to Alexandria. This escalation in autocratic behavior alarmed the Senate, leading to Caligula’s assassination at the hands of his Praetorian Guard. Instead of restoring the Republic, the Senate had to accept Caligula’s uncle – Claudius – as a new emperor.

4. Claudius: The Unexpected Hero

Emperor Claudius (41 – 54 CE) often takes a backseat to his more controversial predecessor and successor. Yet, Claudius, despite his lack of experience and physical disability, proved to be a capable ruler. He oversaw the conquest of Britain, refilled the imperial coffers depleted by his predecessor, and built roads, aqueducts, and canals across the Empire.  

Claudius also tackled social and economic issues. He expanded the port of Ostia to alleviate grain shortages in Rome and reformed the grain dole system, ensuring the provision of wheat to Roman citizens at subsidized rates. Having a personal interest in law, Claudius presided at public trials and issued a myriad of edicts. However, his efforts to combat corruption incited the ire of the nobility, particularly the senators with vested interests in the status quo.

To shore his position, Claudius resorted to extreme measures, killing or exiling several senators, an act earning him the hostility of the ancient writers, many of them members of the Senate. In addition, the emperor’s reign was marred by personal issues, including his marriage to manipulative Messalina and his ambitious niece, Agrippina the Younger. The circumstances surrounding Claudius’ death in 54 CE suggest a possible poisoning, likely orchestrated by Agrippina to ensure the succession of her son, Nero.

Nero (54 – 68 CE) remains one of the most controversial Roman emperors. His reign began auspiciously with tax cuts, public games, and patronage of arts. However, Nero’s youth and inexperience were exploited by his ambitious mother, Agrippina the Younger. The harmonious mother–son joint rulership did not last long and became increasingly strained as Agrippina opposed Nero’s affair with Poppaea Sabina. Perhaps at Poppaea’s instigation, Nero decided to eliminate his mother, which he did after several unsuccessful attempts in 59 CE. 

If matricide was not enough, Nero’s relationship with the Senate was further strained by the death of Empress Poppaea, rumored to be caused by Nero himself, and the emperor’s personal involvement in artistic pursuits and public competitions, deemed unbecoming for the ruler. While the senators despised the emperor, the people adored Nero, considering him their champion. 

However, the Great Fire of Rome of 64 AD was a turning point. Although Nero was unjustly blamed for the fire, the speedy construction of his lavish palace – the Domus Aurea – on the city’s smoldering ruins undermined the emperor’s power and influence. Nero’s estrangement from the Senate and the military led to a revolt, culminating with the emperor’s forced suicide in 68 CE. The ancient sources, written by senators, tarnished the emperor’s name for posterity, and the early Christian historians cast the emperor as a first persecutor, making Nero an Antichrist. 

6. Vespasian: The New Order

Emperor Vespasian (69 – 79 CE) was the first Roman emperor who was not a member of the Julio Claudian dynasty. He was also the first emperor to seize the throne by force. Vespasian spent his reign working tirelessly to restore the Roman Empire’s power and stability following the chaotic “Year of the Four Emperors.”  

Vespasian had no blood relation to his predecessors, which led him to pass the Lex de imperio Vespasiani, a law that conferred imperial powers upon him, a law which would be used by all Roman emperors to legitimize their claim. The new emperor also began a propaganda campaign to legitimize his rule, minting coins praising Vespasian’s restoration of peace, and the support of the army. Emperor Vespasian stabilized finances, funding a vast building program, including the construction of the Colosseum. 

During Vespasian’s reign, Rome continued its territorial expansion, particularly in Britain, through the conquests of General Agricola. He also continued to fight the Jewish revolt in Judea, which was fully suppressed by his son and heir, future emperor Titus, in 73 CE with the fall of Masada fort. Lastly, by establishing the new Flavian dynasty, Vespasian secured a prosperous future for the Roman Empire. 

Emperor Trajan (98 – 117 CE) is undoubtedly one of the greatest Roman emperors. During his reign, the Roman Empire reached its apex, expanding its boundaries into regions of Dacia, Arabia, Armenia, and Mesopotamia, briefly reaching the Persian Gulf. His most significant conquest, the Dacian War, is immortalized on the famed Trajan’s Column, still standing in the heart of Rome. 

Trajan, the first emperor originating out of Italy (Spain), was more than a brilliant military leader. His legions secured vital trade routes and increased the Empire’s wealth, allowing for a grandiose building program in Rome and the provinces. Trajan also implemented numerous social reforms; he expanded the grain dole system to provide food at subsidized rates and introduced the alimenta, a welfare program aimed at supporting orphans and the elderly. No wonder the Roman Senate awarded Trajan the honorific title “Optimus” – “the best” emperor. 

Despite his accomplishments, Trajan’s reign was not without challenges. Roman expansionism strained resources, and rebellions occasionally flared up in conquered territories. However, Trajan’s ability to navigate these issues and maintain stability solidified his legacy. Most importantly, Trajan continued the policy of his predecessor, Emperor Nerva, by adopting his heir, the future emperor Hadrian, thus ensuring the peaceful transfer of power. 

8. Hadrian: The Builder and Traveler

Marked as Trajan’s successor, Hadrian (117 – 138 CE) took the purple in 117 CE. His reign is noted for its relative peace and stability, the only exception being the Bar Kokhba revolt in Judea in 132 CE. A Spaniard like Trajan, Hadrian spent most of his reign out of Rome, extensively touring the provinces and fortifying the Empire’s vast frontier. 

Recognizing the overextension of Rome’s limes, Hadrian strategically repositioned the legions to more defensible locations along the Rhine, Danube, and Euphrates rivers. In the absence of a natural barrier, the emperor commissioned the famed Hadrian’s Wall in Britain – a massive fortification demarcating the Empire’s northern boundary. His architectural legacy also includes the Pantheon in Rome, built by Agrippa but rebuilt by Hadrian, as well as the city of Antinoöpolis in Egypt, founded in memory of his beloved companion Antinous, who drowned in the Nile. 

Hadrian was a patron of arts and admirer of Greek culture, which influenced his building program and the cultural life of the Empire. In Tivoli, near Rome, Hadrian built Villa Adriana – an opulent villa and a veritable museum filled with artworks from, or inspired by, the Hellenistic world. While Hadrian clashed with the Senate, even putting four senators to death, his reign is considered a golden era in Roman history. 

Marcus Aurelius (161 – 180 CE), the last of the “Five Good Emperors”, faced constant military threats, a devastating plague, and internal rebellions. Initially, he shared the throne with his adoptive brother Lucius Verus, who successfully campaigned against Parthia in the East. However, the Antonine plague claimed Verus’ life in 169, wreaking havoc on the Empire’s economy and manpower. To make things worse, the collapse of the Danubian limes left the imperial heartland – Italy – exposed to the barbarian attacks.

Marcus Aurelius spent most of his reign on military campaigns, battling the Germanic tribes like the Quadi and Marcomanni. The Marcomannic Wars (166 to 180 CE), were brutal and costly, putting severe pressure on the embattled emperor. In response to the demands of war and crisis management, emperor Marcus Aurelius began promoting both army officers and civilian administrators on the basis of merit and ability rather than birth and class, thus fostering social mobility and expanding the recruitment pool, allowing for greater flexibility in the future.

Despite the constant challenges, Marcus Aurelius remained a steadfast Stoic. His personal writing, “The Meditations”, composed during the emperor’s military campaigns, remains a testament to Marcus Aurelius’ philosophical reflection on life, duty, and mortality, a rare glimpse into the mind of the Roman emperor wrestling with the burden of command and existential challenges. 

10. Commodus: The Gladiator

In sharp contrast with his competent father, Marcus Aurelius, Commodus (177 – 192 CE) has gone in history as one of the most controversial Roman emperors. Commodus was the first emperor who became that by birth, first as a co-emperor with his father and then becoming sole ruler after Marcus Aurelius died in 180 CE. His reign is often seen as a period of decline, marking the end of the era of the “Five Good Emperors” and the beginning of increased instability within the Roman Empire.

Commodus’ fascination with gladiatorial combat, in which he participated, did not sit well with the Roman Senate, who perceived it as both a threat to their authority and a mockery of Roman values. However, Commodus’ fights in the arena could be a deliberate approach to denigrate the senators while getting closer to the people who adored their down-to-earth emperor. His identification with Hercules could also have been part of the emperor’s legitimization, following the precedent established by the Hellenistic god-kings.

However, the emperor’s strategy backfired, resulting in his assassination in 192 CE. This event not only ended the Nerva-Antonine dynasty but also plunged the Roman Empire into another bloody civil war, the chaotic period known as the “Year of the Five Emperors.” 

Septimius Severus (193 – 211 CE) was a Roman emperor who left an indelible mark on the Empire by militarizing the imperial dynasty. Born in North Africa, Septimius Severus rose to power amid the tumultuous “Year of the Five Emperors,” ultimately prevailing in the civil war that followed the assassination of Commodus. His reign was characterized by a significant shift towards a military monarchy, where the emperor’s authority was closely tied to the support of the army. He also sanctified the imperial family, setting a precedent to become a fourth-century trend.

Septimius Severus tolerated the Senate but made it clear that the army was the basis of his power. The emperor raised the soldiers’ salary, the first in the century, and increased the number of legions from 30 to 33. He also led several successful military campaigns, defeating Parthia and sacking Ctesiphon. Additionally, he extended the Roman territory in Africa, expanding and refortifying the entire southern frontier. 

On his deathbed, Septimius Severus famously told his two sons and heirs, Geta and Caracalla: “Enrich the soldiers, forget everyone else.” However, by laying all power in the hands of the soldiers, Severus made a risky move. The soldiers were powerful backers, but only for the emperors who proved to be competent commanders. When the Severan dynasty failed to produce a capable leader, the soldiers took the power themselves, kickstarting the Third Century crisis. 

12. Caracalla: The Ruthless Leader

Emperor Caracalla (198 – 217 CE), the elder son of Septimius Severus, was a model representative of the new military monarchy. Both he and his brother Geta inherited the throne, but following the power struggle, Caracalla had his brother killed and removed all traces of his memory (damnatio memoriae) from coins, sculptures, paintings, and the rest of iconography. He also purged all of his brother’s supporters.

Caracalla’s most notable act, the Constitutio Antoniana in 212 CE, extended Roman citizenship across the Empire, a revolutionary move as it integrated a vast and diverse population under a single legal status. The edict, however, had financial motives, as it broadened the tax base and to fund military salaries or grandiose projects like the Baths of Caracalla in Rome.  

Caracalla’s reign was marked by his close association with the military; he increased the soldiers’ pay and elevated their societal standing. Preferring the company of his troops, he spent much of his time in military encampments and on campaigns, never returning to Rome. The emperor’s moniker, “Caracalla,” was derived from the Gallic hooded military tunic he habitually wore and made fashionable. Although successful as an emperor and general, Caracalla’s ruthlessness caused discontent among his officers, culminating in the emperor’s assassination on his way to Parthia.

The brief reign of Maximinus Thrax (235 – 238 CE) marked a new chapter in the Roman Empire’s history – known as the Third Century Crisis – a time when the Roman Empire was beset by external invasions, internal civil wars, and a succession of military rulers. The period is also known as the reign of the barrack emperors, or soldatenkaiser. 

Maximinus Thrax was a lowborn soldier who quickly climbed the ranks and, after the army murdered young Alexander Severus, seized the throne in 235. Maximinus was a stark contrast to his predecessors. The sources tell us he was a whopping 2.4 meters tall (!) brute of enormous strength. According to Herodian, Maximinus was a Thraco-Roman, while the unreliable Historia Augusta calls him a barbarian.  

Despite his military victories, the soldier-emperor’s lack of experience in governing the vast Empire, his heavy-handed tactics, and his hostility to the Senate alienated the elites, fueling political instability. In 238 CE, during a failed campaign in Italy, the emperor and his son were assassinated by their own troops. Maximinus Thrax’s reign, though brief, is seen as a turning point, foreshadowing the decline of central authority and the rise of military strongmen in the coming decades.

14. Valerian: A Captive

Emperor Valerian (253 – 260 CE) was a rare exception in the era of low-born soldier emperors. An aristocrat and senator, Valerian ascended the throne after a brief civil war and was proclaimed emperor by his troops, along with his son Gallienus, in 253. 

Valerian had little time to govern as the Eastern frontier was under attack by Rome’s new nemesis, the Sassanid Empire. Valerian quickly marched eastwards, at the head of the army, only suffer a decisive defeat in the Battle of Edessa in 260 CE. To make matters worse, Valerian was captured, becoming the first Roman emperor captured by the foreign enemy. 

However, the story of the emperor being used as a human stool for victorious Shapur I, or being forced to swallow molten gold or being flayed alive, are probably exaggerations, tales embellished by early Christian historians, portraying Valerian as a persecutor of Christians. Whatever his fate, Valerian’s demise left the throne to his son Gallienus, who had to deal with yet another revolt, perhaps the most dangerous of all, as the Roman Empire broke apart.

Emperor Aurelian (270 – 275 CE) stands out as a pivotal figure in Roman history, renowned for rescuing the Empire from the verge of collapse. Aurelian inherited a deeply fractured state, with the breakaway states of the Gallic Empire of Postumus in the West and the Palmyran Empire of Zenobia in the East threatening Rome’s unity and stability. Luckily, Aurelian was a man ready for the task, who, in his brief reign, eliminated all the threats, and reunified the Roman Empire as the sole ruler.

A lowborn soldier from the Illyricum, Aurelian advanced quickly through the ranks during the reigns of Gallienus and Claudius Gothicus. In 270, the army elected Aurelian as the emperor, just in time to defeat the Juthungi attack on Italy. To strengthen the vulnerable borders, Aurelian abandoned Dacia. He also fortified Rome with the mighty Aurelian Walls. Most importantly, Aurelian defeated both breakaway states of Gaul and Palmyra, reuniting the Roman Empire and becoming “Restitutor Orbis” – the “Restorer of the Roman World.”

Aurelian also initiated monetary and religious reforms, attempting to stabilize the Roman currency, and promote the worship of Sol Invictus – the “Unconquered Sun.” Among sweeping reforms that transformed the Empire, Aurelian elevated the imperial office, becoming “dominus et deus,” master and god, laying the foundation for the absolutist rule defining the later Roman Empire. Despite his numerous achievements, Aurelian could not end the vicious circle of internal unrest, falling victim to a plot while leading the army to the Sassanid campaign in 275 CE.

16. Diocletian: The Reformer

Like Aurelian, emperor Diocletian (284 – 305 CE) came from Illyricum, rising through the ranks and taking the purple with the army’s support. His reign was marked by significant stability and longevity, a stark contrast to the short tenures of many of his predecessors. Diocletian managed such a feat by redefining imperial rule – instituting a system known as the Tetrarchy.

Recognizing the vastness and vulnerability of the Roman Empire, Diocletian divided the Roman Empire into four administrative regions, each ruled by a senior emperor (Augustus) with a designated heir (Caesar). The Tetrarchy aimed to provide more efficient governance and military defense and ensure a smoother transition of power. The first two Augusti were Diocletian and his fellow officer Maximian, while the juniors were Galerius and Constantius.  

With the Empire stable and secure, Diocletian enacted sweeping reforms, strengthening the empire’s finances, reorganizing the administration and the army, redefining the emperor’s office, and introducing more centralized, autocratic rule. The Tetrarchy, however, fractured soon after Diocletian’s voluntary abdication in 305 CE (the first for a Roman emperor) due to power struggles among his successors.

Emperor Constantine the Great (306 – 337 CE) rightly deserves his moniker. Besides perhaps Augustus, no Roman emperor made such a pivotal change, charting the course of the Empire for the next thousand years. Constantine played a central role in dismantling Diocletian’s Tetrarchy, plunging the Empire into a series of civil wars, from which he emerged as the sole victor. 

The most important of Constantine’s military achievements is his victory over Maxentius’ in the Battle of the Milvian Bridge in 312. Constantine’s vision before the battle (“In Hoc Signo Vinces”) eventually led to the Edict of Milan in 313, by which Christianity became the permitted religion and one of the main imperial religions. Constantine I converted to Christianity only on his deathbed, but he laid a strong foundation for Christianizing the Roman Empire, Europe, and eventually the world.

Another fateful decision of Constantine was the foundation of Constantinople, the new imperial capital in 330, which shifted the Empire’s center eastwards, assuring its survival for another millennium. Constantine also enacted a series of reforms: He restructured the Roman government, separated military and civil authority, reorganized the army, and introduced solidus – the “dollar of the Middle Ages.” Lastly, Constantine the Great established a new imperial dynasty, further strengthening the Roman Empire and making it a fourth-century superpower. 

18. Julian: The Last Pagan Emperor

Emperor Julian, known as Julian the Apostate (361 – 363 CE), was the last Roman emperor who tried (but failed) to turn the clock back and restore paganism in a rapidly Christianizing Empire. One of the few members of the imperial family to survive the purges and civil wars of his cousin – emperor Constantius II – Julian was appointed a junior emperor (Caesar) and given command over the army of Gaul. There, he earned his troops’ loyalty through victories against the Alamanni, leading to Julian’s acclamation as emperor.

Constantius’ sudden death in 361 spared the Roman Empire another civil war, leaving Julian the sole ruler. A scholar enamored with Hellenistic culture, Julian enacted sweeping reforms to simplify the imperial court and reduce expenses. However, the philosopher-emperor’s most significant act was to revive paganism, restoring temples and rituals, and appointing pagans to prominent positions, thereby diminishing Christianity’s influence. This led to an open conflict with the Christian elites, notably the people of Antioch, who mocked their pagan emperor.

Perhaps to legitimize his usurpation and unite a deeply divided army, Julian embarked on a Sassanid campaign in 363 CE. Although initially successful, Julian’s Persian expedition ultimately failed, leading to his death in battle. The emperor’s demise was followed by the reversal of all his pagan policies, with Christianity solidifying its dominance in subsequent years. 

Emperor Theodosius I (379 – 395 CE) ascended the throne during a period of chaos. The catastrophic defeat at Adrianople in 378 and the death of Emperor Valens left the Balkans vulnerable to Gothic incursions. With limited resources and a depleted army, Theodosius struggled to repel the Goths. Ultimately, he permitted these “barbarians” to settle as autonomous allies, known as the foederati, within the imperial territory. This decision had profound and lasting consequences, contributing to the Empire’s gradual decline in the following decades.

As a devout Christian ruler, Theodosius convened the First Council of Constantinople in 381, which affirmed the Nicene Creed and condemned Arianism. His most pivotal act, however, was declaring Christianity the official religion of the Empire, effectively outlawing pagan practices. This decree marked a significant turning point, solidifying Christianity’s dominance within the Roman Empire.

Theodosius achieved a durable peace with the Sassanian Empire, but his aspirations for a dynastic legacy led to two devastating civil wars, depleting Rome’s already strained military resources. Theodosius’ unexpected death in 395 left the Empire in the hands of his young sons, Honorius and Arcadius. However, real power lay with powerful military commanders and bureaucrats like Stilicho and Rufinus. While in the East, the emperors eventually restored their authority, in the West, imperial power steadily declined, culminating in the disintegration of the Western Roman Empire.

20. Constantine XI – The Last Roman Emperor

Emperor Constantine XI Palaiologos (1449 – 1453 CE) was the last Roman Emperor, presiding over the remnants of a once mighty state, also known as the Byzantine Empire. Inheriting a realm on the verge of collapse, Constantine faced the formidable Ottoman Empire, poised to seize Constantinople. Born into the last imperial dynasty – the Palaiologoi – Constantine had to deal with challenges from the very start, including the need to secure internal stability and bolster imperial defenses against the Ottomans, led by the ambitious Sultan Mehmed II.

During his brief reign, emperor Constantine worked tirelessly, sought alliances with Western powers and the Pope, even recognizing the Catholic Church’s supremacy, which angered his subjects. Despite his diplomatic efforts, Western assistance was limited, and the Empire had to face alone the overwhelming military might of Ottoman forces.

In the face of these insurmountable odds, outnumbered and aided only by a few Westerners, like Genovese general Giustinianni, Constantine XI led the defense of Constantinople during the city’s final siege in May 1453. The last Roman emperor fought valiantly alongside his men until the Theodosian Walls fell to the Ottoman cannons. Constantine’s death in battle marked not only the end of his reign but also the end of the unbroken line of emperors going back all the way to Augustus. It also marked the end of the Roman Empire and the dawn of a new era.