The most straightforward theory for Western Rome’s collapse pins the fall on a string of military losses sustained against outside forces. Rome had tangled with Germanic tribes for centuries, but by the 300s “barbarian” groups like the Goths had encroached beyond the Empire’s borders. The Romans weathered a Germanic uprising in the late fourth century, but in 410 the Visigoth King Alaric successfully sacked the city of Rome.

The Empire spent the next several decades under constant threat before “the Eternal City” was raided again in 455, this time by the Vandals. Finally, in 476, the Germanic leader Odoacer staged a revolt and deposed Emperor Romulus Augustulus. From then on, no Roman emperor would ever again rule from a post in Italy, leading many to cite 476 as the year the Western Empire suffered its death blow.

Even as Rome was under attack from outside forces, it was also crumbling from within thanks to a severe financial crisis. Constant wars and overspending had significantly lightened imperial coffers, and oppressive taxation and inflation had widened the gap between rich and poor. In the hope of avoiding the taxman, many members of the wealthy classes had even fled to the countryside and set up independent fiefdoms.

At the same time, the empire was rocked by a labor deficit. Rome’s economy depended on slaves to till its fields and work as craftsmen, and its military might had traditionally provided a fresh influx of conquered peoples to put to work. But when expansion ground to a halt in the second century, Rome’s supply of slaves and other war treasures began to dry up. A further blow came in the fifth century, when the Vandals claimed North Africa and began disrupting the empire’s trade by prowling the Mediterranean as pirates. With its economy faltering and its commercial and agricultural production in decline, the Empire began to lose its grip on Europe.

The fate of Western Rome was partially sealed in the late third century, when Emperor Diocletian divided the Empire into two halves—the Western Empire seated in the city of Milan, and the Eastern Empire in Byzantium, later known as Constantinople. The division made the empire more easily governable in the short term, but over time the two halves drifted apart. East and West failed to adequately work together to combat outside threats, and the two often squabbled over resources and military aid.

As the gulf widened, the largely Greek-speaking Eastern Empire grew in wealth while the Latin-speaking West descended into an economic crisis. Most importantly, the strength of the Eastern Empire served to divert Barbarian invasions to the West. Emperors like Constantine ensured that the city of Constantinople was fortified and well guarded, but Italy and the city of Rome—which only had symbolic value for many in the East—were left vulnerable. The Western political structure would finally disintegrate in the fifth century, but the Eastern Empire endured in some form for another thousand years before being overwhelmed by the Ottoman Empire in the 1400s.

At its height, the Roman Empire stretched from the Atlantic Ocean all the way to the Euphrates River in the Middle East, but its grandeur may have also been its downfall. With such a vast territory to govern, the empire faced an administrative and logistical nightmare. Even with their excellent road systems, the Romans were unable to communicate quickly or effectively enough to manage their holdings.

Rome struggled to marshal enough troops and resources to defend its frontiers from local rebellions and outside attacks, and by the second century, the Emperor Hadrian was forced to build his famous wall in Britain just to keep the enemy at bay. As more and more funds were funneled into the military upkeep of the empire, technological advancement slowed and Rome’s civil infrastructure fell into disrepair.

If Rome’s sheer size made it difficult to govern, ineffective and inconsistent leadership only served to magnify the problem. Being the Roman emperor had always been a particularly dangerous job, but during the tumultuous second and third centuries it nearly became a death sentence. Civil war thrust the empire into chaos, and more than 20 men took the throne in the span of only 75 years, usually after the murder of their predecessor.

The Praetorian Guard—the emperor’s personal bodyguards—assassinated and installed new sovereigns at will, and once even auctioned the spot off to the highest bidder. The political rot also extended to the Roman Senate, which failed to temper the excesses of the emperors due to its own widespread corruption and incompetence. As the situation worsened, civic pride waned and many Roman citizens lost trust in their leadership.

The Barbarian attacks on Rome partially stemmed from a mass migration caused by the Huns’ invasion of Europe in the late fourth century. When these Eurasian warriors rampaged through northern Europe, they drove many Germanic tribes to the borders of the Roman Empire. The Romans grudgingly allowed members of the Visigoth tribe to cross south of the Danube and into the safety of Roman territory, but they treated them with extreme cruelty.

According to the historian Ammianus Marcellinus, Roman officials even forced the starving Goths to trade their children into slavery in exchange for dog meat. In brutalizing the Goths, the Romans created a dangerous enemy within their own borders. When the oppression became too much to bear, the Goths rose up in revolt and eventually routed a Roman army and killed the Eastern Emperor Valens during the Battle of Adrianople in A.D. 378. The shocked Romans negotiated a flimsy peace with the barbarians, but the truce unraveled in 410, when the Goth King Alaric moved west and sacked Rome. With the Western Empire weakened, Germanic tribes like the Vandals and the Saxons were able to surge across its borders and occupy Britain, Spain and North Africa.

The decline of Rome dovetailed with the spread of Christianity, and some have argued that the rise of a new faith helped contribute to the empire’s fall. The Edict of Milan legalized Christianity in 313, and it later became the state religion in 380. These decrees ended centuries of persecution, but they may have also eroded the traditional Roman values system. Christianity displaced the polytheistic Roman religion, which viewed the emperor as having a divine status, and also shifted focus away from the glory of the state and onto a sole deity.

Meanwhile, popes and other church leaders took an increased role in political affairs, further complicating governance. The 18th-century historian Edward Gibbon was the most famous proponent of this theory, but his take has since been widely criticized. While the spread of Christianity may have played a small role in curbing Roman civic virtue, most scholars now argue that its influence paled in comparison to military, economic and administrative factors.

For most of its history, Rome’s military was the envy of the ancient world. But during the decline, the makeup of the once mighty legions began to change. Unable to recruit enough soldiers from the Roman citizenry, emperors like Diocletian and Constantine began hiring foreign mercenaries to prop up their armies. The ranks of the legions eventually swelled with Germanic Goths and other barbarians, so much so that Romans began using the Latin word “barbarus” in place of “soldier.”

While these Germanic soldiers of fortune proved to be fierce warriors, they also had little or no loyalty to the empire, and their power-hungry officers often turned against their Roman employers. In fact, many of the barbarians who sacked the city of Rome and brought down the Western Empire had earned their military stripes while serving in the Roman legions.