The Roman Empire began in 27 BCE when Augustus became its first emperor, reached its largest territorial extent under Trajan in 117 CE, and ended in the west in 476 CE when Odoacer deposed the last emperor, Romulus Augustulus. Historians point to a mix of military, economic, political, and external pressures over roughly two centuries as the causes of its decline.
Ancient Rome's political history is usually divided into three phases: the Kingdom, the Republic, and the Empire. According to Roman tradition recorded by the first-century BCE scholar Marcus Terentius Varro, the city was founded on 21 April 753 BCE by the legendary twins Romulus and Remus, a story that historians regard as a later invention rather than a factual record, according to History.com.
The rise of the Empire: 27 BCE
The Republic ended after decades of civil war. Octavian, later known as Augustus, defeated his rival Mark Antony at the Battle of Actium in 31 BCE and became Rome's first emperor in 27 BCE, according to World History Encyclopedia. Augustus initiated the Pax Romana, a roughly two-century period of relative internal peace and expansion across the Mediterranean world.
Between 96 and 180 CE, five successive rulers — Nerva, Trajan, Hadrian, Antoninus Pius, and Marcus Aurelius, often referred to as the "Five Good Emperors" — presided over a period of stability and continued military and administrative consolidation, per World History Encyclopedia.
Peak extent under Trajan, 117 CE
The Empire reached its greatest territorial size around 117 CE under Emperor Trajan. According to figures from UNRV (United Nations of Roma Victrix, a dedicated Roman-history reference site), the Empire covered approximately 2.2 million square miles (about 5.7 million square kilometers), stretching from Britain in the northwest to Armenia in the east, and from the North Sea to the Red Sea.
Population estimates for the Empire at its height vary among historians. UNRV states that around 60 million people held Roman citizenship at the peak, while estimates for the total number of people living within the Empire's borders during the imperial period range as high as 120 million.
Division and the start of decline
By the late third century, the Empire had become difficult to govern and defend as a single unit. Emperor Diocletian, who came to power in 284 CE, divided administration between a Western and an Eastern half around 285 CE, according to World History Encyclopedia. History.com notes that this division ultimately left the wealthier, better-fortified Eastern half better positioned to deflect invasions, while the Western Empire and Italy bore more pressure from migrating peoples.
Two military disasters marked the acceleration of Western decline. In 378 CE, at the Battle of Adrianople, Gothic forces who had crossed the Danube frontier destroyed a large part of the Roman army and killed Emperor Valens, according to World History Encyclopedia. In 410 CE, the Gothic leader Alaric sacked the city of Rome itself over three days, reportedly taking what World History Encyclopedia describes as roughly two tons of gold and thirteen tons of silver before withdrawing.
The end in the West: 476 CE
By 475 CE, the Western Roman Empire's effective control had shrunk largely to Italy, according to World History Encyclopedia. On 4 September 476 CE, the Germanic military leader Odoacer deposed the sixteen-year-old Western emperor Romulus Augustulus without significant military resistance — an event historians widely treat as the conventional end point of the Western Roman Empire, though the source notes the state had already been disintegrating for decades.
Although the Western Empire ended, the Eastern Roman Empire — later known as the Byzantine Empire — continued for several more centuries, according to World History Encyclopedia.
Why did Rome fall? Historians disagree
There is no single agreed cause. History.com lists eight commonly cited factors: barbarian invasions; economic troubles tied to warfare costs, inflation, and a shrinking supply of enslaved labor after territorial expansion stopped; the division of the Empire, which diverted invasions westward; overexpansion and the administrative and military costs of defending vast borders; political instability, including more than 20 emperors in 75 years during the second and third centuries, often removed by assassination; pressure from Hunnic migrations pushing Germanic peoples toward Roman borders; the rise of Christianity, which the eighteenth-century historian Edward Gibbon blamed for weakening traditional Roman civic values, though many modern historians dispute this; and the weakening loyalty of legions increasingly staffed by foreign mercenaries.
The decline and fall were only in the west, as the Eastern Roman Empire persisted for centuries.
— World History Encyclopedia
World History Encyclopedia similarly emphasizes that no single cause explains the collapse, pointing instead to a compounding of lost tax revenue, military overstretch, migration pressure, and political dysfunction across roughly two centuries rather than a sudden event.
Roman Empire: key dates
Event
Date
Source
Legendary founding of Rome
753 BCE (traditional)
History.com
Augustus becomes first emperor
27 BCE
World History Encyclopedia
Peak territorial extent under Trajan
117 CE
UNRV
Battle of Adrianople
378 CE
World History Encyclopedia
Sack of Rome by Alaric
410 CE
World History Encyclopedia
Fall of the Western Empire
4 September 476 CE
World History Encyclopedia / History.com
Questions
When did the Roman Empire officially begin?
The Roman Empire began in 27 BCE, when Octavian took the title Augustus and became Rome's first emperor after the civil wars that ended the Republic, according to World History Encyclopedia.
When was the Roman Empire at its largest?
The Empire reached its greatest territorial extent around 117 CE under Emperor Trajan, covering roughly 2.2 million square miles across Europe, North Africa, and the Near East, according to UNRV.
When and how did the Roman Empire fall?
The Western Roman Empire is conventionally dated to have ended on 4 September 476 CE, when the Germanic leader Odoacer deposed Emperor Romulus Augustulus without major resistance, according to World History Encyclopedia and History.com.
What caused the fall of Rome?
Historians cite a combination of factors rather than one cause, including barbarian invasions, economic strain, political instability, military overstretch, the empire's division, Hunnic migration pressure, and the weakening loyalty of legions, according to History.com and World History Encyclopedia.
Did the whole Roman Empire fall in 476 CE?
No. Only the Western Roman Empire ended in 476 CE; the Eastern Roman Empire, later called the Byzantine Empire, continued for several more centuries, according to World History Encyclopedia.
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