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Few periods in history have had a greater impact on humankind than that of ancient Rome. While its influence on western civilization, in particular, has been ubiquitous, its remnants can be found virtually everywhere, from our calendar and political systems to our alphabet. The more-than-1,000-year span of influence that began with the founding of Rome in 753 B.C. has left an indelible mark on the world.

So who exactly left an indelible mark on ancient Rome?

From its inception to its collapse in 476 A.D., ancient Rome had three distinct periods: Regal Rome, (753–509 B.C.), when monarchs ruled; Republican Rome (509–27 B.C.), when Roman elected its governors; and Imperial Rome (27 B.C.–476 A.D.), when a supreme ruler oversaw the empire, and in early years did so alongside the elected senate. Over that time, Rome was ruled by scores of kings, dictators and emperors who expanded it from a small city to an empire spanning nearly 2 million square miles and consisting of, historians estimate, anywhere from 50 to 90 million inhabitants.

These rulers, often as innovative and ingenious as they were brutal and corrupt, spanned the gamut—from teenagers and impotent leaders barely able to hold court for months to era-defining emperors responsible for molding at least part of the world today as we know it. Here are a few of the most influential.

READ MORE: How Ancient Rome Thrived During Pax Romana

Gaius Julius Caesar (reign from 49 B.C. to 44 B.C.)

Death of Julius Caesar

The death of Julius Caesar.

Technically, as the last ruler of Rome’s Republican era, Gaius Julius Caesar was never recognized as an emperor. But it’s impossible to tell the story of Rome (or its eventual transition from a republic to an empire, without mentioning Julius Caesar. Aside from being a successful general, conquering Spain and Gaul—feats that greatly expanded the size, power and wealth of Rome—Caesar enacted a number of foundational reforms that would set up the oncoming Roman Empire. As leader of the Roman Republic, Caesar increased the size of the senate to represent more Roman citizens, established the Julian calendar (the 365-day, 12-month calendar still in use worldwide), granted Roman citizenship to all those living under Roman rule and redistributed wealth among the poor. These reforms made Caesar increasingly popular with Rome’s commoners while alienating him from its elite (and leading to his eventual infamous assassination). After his murder at the hands of dozens of members of the senate, Rome officially transitioned from a democracy to an imperial society.

READ MORE: How Julius Caesar's Assassination Triggered the Fall of the Roman Republic

Caesar Augustus (Reign: 27 B.C. to 14 A.D.)

Augustus born Gaius Octavius Thurinus known as Gaius Julius Caesar Octavianus 63BC - 14AD First emperor of the Roman Empire

Gaius Octavius Thurinus

Gaius Octavius Thurinus, also known as Octavian or “Augustus,” served as the first official emperor of the Roman Empire, and is often seen by historians as the greatest. The emperor (for whom the month of “August” is named) introduced the period of peace known as the Pax Romana, which saw the Roman economy, agriculture and arts flourish. During that period of relative peace, Augustus also established a number of reforms—including tax incentives for families with more than three children and penalties for childless marriages—that helped the Roman population grow. An aggressive builder, he also oversaw the construction and rehabilitation of many of Rome’s great temples and the strengthening of its legendary aqueduct system.

READ MORE: 8 Things You May Not Know About Augustus

Tiberius (Reign: 14 to 37 A.D.)

Tiberius Claudius Caesar Augustus Germanicus or Claudius I 10BC -54AD Roman Emperor of the Julio-Claudian dynasty

Tiberius Caesar Augustus

In ancient Rome, few emperors were better at acquiring land for the empire than Tiberius Caesar Augustus. Rome’s second emperor owes his place on this list solely due to his military conquests. As an emperor and politician, Tiberius is largely considered to have been uninterested in the job and not shy in showing that disinterest. (Roman philosopher Pliny the Elder called him “the gloomiest of men.”) When it came to conquering neighboring lands and expanding Rome’s territory, however, few were better. During his reign, he oversaw one of the greatest military expansions in ancient Rome’s history, widening the empire’s boundaries deep into present-day Croatia and Germany.

READ MORE: 8 Ways Roads Helped Rome Rule the Ancient World

Vespasian (reign: 69 to 79 A.D.)

Vespasian Titus Flavius Sabinus Vespasianus AD 9 - 79 Roman Emperor

Titus Flavius Vespasianus

After the tyrannical reign of Emperor Nero, Rome found itself in a crisis of instability. So much so that during the 12-month span following Nero’s death, the empire had four different rulers (known as the “Year of the Four Emperors”). It wasn’t until Titus Flavius Vespasianus ascended the throne that stability and prosperity returned to Rome, setting the nation back on track. During his reign, Vespasian helped reform the financial system and began many ambitious construction projects, most notably the Colosseum. Vespasian was also the first Roman emperor ever to be succeeded by his son. That father-son handoff would lay the groundwork for the Flavian Dynasty, a near three-decade period of fiscal and cultural prosperity.

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Trajan (reign: 98 to 117 A.D.)

Emperor Trajan among the soldiers.

Emperor Trajan among the soldiers.

Often in the conversation for “greatest Roman emperor” by historians, Marcus Ulpius Traianus was the second Roman emperor in the Nerva-Antonine dynasty commonly referred to as Rome’s “Golden Age.” Bolstered by one of the greatest military expansions in Roman history, Trajan’s reign marked the peak of Rome’s geographic expansion, as it covered nearly 1.7 million kilometers of territory in Europe, Africa and Asia and boasted nearly 57 million people. In addition to his military successes, Trajan also oversaw many ambitious building projects, including the still-standing architectural marvel, Trajan’s Column. He also expanded Augustus’ financial aid programs for poor Roman citizens, in turn providing one of the earliest examples in history of a federal welfare program.

READ MORE: How Far Did Ancient Rome Spread

Hadrian (reign: 117 to 138 A.D.)

A defensive fortification in the Roman province of Britannia, begun in 122 AD in the reign of the emperor Hadrian, known as Hadrian's Wall.

A defensive fortification in the Roman province of Britannia, begun in 122 AD in the reign of the emperor Hadrian, known as Hadrian's Wall.

Publius Aelius Hadrianus claims a spot as one of Rome’s most influential emperors for his ability to secure Rome and its borders and the unprecedented engineering prowess he displayed while doing so. He oversaw construction of Hadrian’s Wall, a 73-mile-long defensive fort—much of which still stands today and is recognized as a British cultural icon. He also leaves the Pantheon, which revolutionized architecture with its innovative construction of shapes built with concrete.

READ MORE: 10 Innovations That Built Ancient Rome

Antoninus Pius (reign: 138 to 161 A.D.)

Titus Aelius Hadrianus Antoninus Pius presided over Rome during one of the civilization’s most peaceful periods. That lack of turmoil afforded Pius the opportunity to focus on advancing on the infrastructure successes and civic reforms of his predecessor Hadrian. His greatest contribution to Roman civilization, however, came through the legal system. As the first Roman emperor to adhere to the concept of “natural law,” Pius instituted a legal system that would serve later as the reference point for many nations developing their own legal systems, including Britain, France and Germany.

Marcus Aurelius (reign: 161 to 180 A.D.)

Marcus Aurelius Antoninus

Marcus Aurelius Antoninus

Known as the “emperor-philosopher,” emperor Marcus Aurelius Antoninus produced writings now considered philosophical canon. A fervent adherent of Stoicism—a Hellenistic school of philosophy that claimed that becoming a clear and unbiased thinker was key to gaining universal reason—the emperor (who was famously portrayed in the Oscar-winning “Gladiator”) is widely regarded as one of history’s most essential philosophers. His book Meditations is largely regarded as a literary masterpiece.

Valerian (reign: 253 to 260 A.D.)

Publius Licinius Valerianus makes the influential list less for what he did than what was done to him. In 260 A.D., after the Battle of Edessa against the Persians, Valerian (a notorious persecutor of Christians) became the first Roman emperor to be taken as a prisoner of war. The unprecedented capture sent shockwaves through the Roman Empire, only to be exacerbated by the fact Valerian was never rescued. The emperor went on to die in captivity under unknown circumstances. Rome’s inability to rescue its own sovereign would deal a seismic blow to the mystique of power the Romans held over the world. And, many historians believe, it would plant the seed in the minds of foreign nations that the previously “unconquerable” nation of Rome could indeed be toppled.

Diocletian (reign 284 to 305 A.D.)

Map of Roman empire under Emperor Diocletian rule, AD 284-305

Map of Roman empire under Emperor Diocletian rule (AD 284-305). 

On one hand, Gaius Aurelius Valerius Diocletianus deserves to be remembered for saving Rome from the “Crisis of the Third Century,” a near 50-year period—marked by civil war, political instability rebellions and invasions—during which the empire nearly collapsed. On the other, some historians believe it was his installation of the “tetrarchy” form of government that might prove his most valuable contribution. Under the tetrarchy, Diocletian mandated that Rome would be ruled by four leaders: an emperor in the west, one in the east (the “Augustus” emperor) and two junior co-emperors (the “Caesars”). The tetrarchy didn’t last, but it did provide the groundwork for the practice of splitting the Roman empire into eastern and western halves, a move that would prove crucial in extending its lifespan.

READ MORE: 6 Civil Wars That Transformed Ancient Rome

Constantine the Great (reign: 306 to 337 A.D.)

Considered by many to be the last western Roman emperor, Constantine I brought many changes that would irrevocably alter the Roman empire. He was the first Roman emperor to convert to Christianity and permanently established religious tolerance for Christianity with his Edict of Milan in 313 A.D. Constantine also built Byzantium (later renamed Constantinople), which would become the empire’s epicenter for the next thousand years and mark the transition into the new epoch known as the Eastern Roman Empire or Byzantine Empire.

READ MORE: 8 Reasons Why Rome Fell

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