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Rise of the Republic II: The Battle of the Allia

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The author is a student of ancient and modern European history.

Read on to learn all about the Battle of the Allia and the ensuing sack of Rome. These events presaged Rome's later intense militarism throughout Italia.

Read on to learn all about the Battle of the Allia and the ensuing sack of Rome. These events presaged Rome's later intense militarism throughout Italia.

Rise of the Republic

Rome was not always a republic. The city was founded as a monarchy and was ruled by seven fabled kings who instilled within the Roman people a love of law, religion, and war-making. When the kings of Rome were toppled by the Roman Senate, the power of the state was transferred to the Consuls, nominally elected leaders of the Roman people. For one hundred years, from the Battle of Lake Regillus to the Siege of Veii, Rome was stricken with a series of internal disorders.

The Conflict of the Orders which arose from the distribution of political power in the wake of deposing the Roman monarchy led to a series of political and economic actions pitting the patricians against the plebeians. These events would dominate Roman life for the time period, while intermittent raids from neighboring tribes caused fleeting moments of unification. While the Conflict of Orders would continue into the 4th century, the Battle of Veii redirected the attention of the Roman Republic.


In the aftermath of the Battle of Veii, Roman society and Rome's international outlook were rewritten. Prior to the Battle of Veii, Rome engaged in warfare on the basis of raiding and pillaging, where they would devastate a neighbor's land, possibly sack their city, and then go home until the next year's conflict.

Veii changed everything. Rome put the men to the sword and took the women and children as slaves. Rome would then colonize Veii. Though this was not the first Roman colony, it was a large Etruscan city, and it reopened Roman involvement in the Etruscan sphere. The destruction of Veii made Rome the preeminent power in central Italy, and it inextricably led to the Battle of the Allia.


Setting the Stage

The Senones were part of a Celtic tribe living in modern-day France from whom a splinter group settled in north-eastern Italy along the Adriatic coast. Several theories abound for why the Gauls migrated south into Italy, and there is probably a little truth in each of them.

The first major theory is recounted by Livy who claimed the Celts migrated out of Gaul because of overpopulation. Under the fabled tribal leader, Bellovesus Gallic tribes traded with the Greek colonists of Marsala and in doing so discovered passages across the Alps. There they made war upon the Etruscans and Umbrians. After defeating them the Celts established their own cities in Italy.

A second theory is that the Celts were hired to fight as mercenaries. Gallic soldiers were considered to be some of the best warriors of the ancient world. Celtic religion held that the soul was reincarnated upon death and ancient writers state that because of this Gallic warriors did not fear death.

Celts were also famous for their iron working, of which the Gallic longsword was well known and feared throughout the ancient world. A willingness to fight to the death and the tools to break enemy formations forged the Gallic people into formidable warriors.

While Rome was advancing north into Etruria after the Battle of Veii, the Senones were pushing south into Etruria from their city of Senigallia in the northeast of Italy. In an era of plunder and raiding, with food a constant restraint to survival, and rich trade cities looking for people to do their fighting for them, either theory is possible.

What is certain is that the Senones laid siege to the Etruscan city of Clusium, a city Rome considered a friend since the expulsion of Tarquin the Proud. The Clusians called to Rome for help, and when Rome answered affirmatively the Gauls broke their siege and pushed south.


Disaster at Allia

Rome answered the Clusian call to war by sending ambassadors to try to negotiate peace, but the ambassadors instead ended up fighting for Clusium against the Senones. This was a serious breach of protocol for the ancient world. Diplomats were typically allowed some length of freedom of movement.

This was sanctioned not only as a political activity but under a sacred oath protected by the gods. By breaking this oath and joining the fighting, the Roman ambassadors committed a sacrilegious act in the eyes of the Celtic tribe.

Rome rallied its armies and marched north, while the Senones under Brennus broke their siege and surged south. Here the historical data becomes unclear. Contemporary sources state that the Roman army had 24,000 to 40,000 men and the Celts had about the same.

Modern historiography disputes these numbers based on the archeological finds of that time period. Estimates for the entire population of Rome at this time based on the size of Rome's territory and archeological finds give a range of 25,000 to 40,000 total people living in Roman territory.

Given how quickly the Roman army was called into action and how many people a population that size could actually muster in men on the field, a more reasonable number of soldiers would be around nine thousand Romans in the Republican Legions plus some Latin allied troops. For their part, the Celtic tribes were a loose tribal confederation and lacked large metropolitan areas to draw soldiers from, and could not have fielded more than twelve thousand warriors.

This would ring true for Plutarch's assessment that the Roman force was not seriously outnumbered but diverged greatly from the numbers provided by contemporary sources like Livy.

While the number of participants is disputed, the events of the battle from both Livy and Diodorus Siculus are in agreement about how the battle was fought. The battle was fought on a small plain abutted by small hills and the Tiber river. As the Roman force had been hastily assembled, there were undisciplined troops in the formation, probably leves who were lightly armed and unarmored.

These troops were moved into the hills to provide the army with a reserve force; however, Brennus, chieftain of the Senones, sent his best men into the hills to engage the leves while the rest of the Celtic line engaged the Roman battle-line in the plains.

Though the battle stalled in the plains, the picked men routed the Roman force in the hills before swinging back into the main battle and slamming into the main Roman force on its flank. As their flanks turned the Roman army shattered and was driven into the Tiber where javelineers and the rough water claimed many Roman lives.

Some of the Roman forces escaped to Veii where they regrouped while only a small handful returned to Rome. The Celtic forces stayed at the site of the battle to loot the dead and take the skulls of the fallen.


Explanations for the Battle

While the Roman army had not yet adopted the Triplex Acies there is no reason to believe that they were fighting in phalanx formation as we understand it to exist in Late Antiquity. Roman armies fought primarily as heavy infantry supported by cavalry at a ratio of one cavalrymen to ten infantrymen. They likely fought primarily with a heavy spear and hoplon shield in this period, much like a hoplite, but certainly not in a closed formation like a sarissa armed phalangite.

Roman soldiers in this battle would not have had time to train for coordinated large regiment movements. In fact, the Roman system of leveeing troops ensured that Roman troops in the Axial Age would never have had enough training to move as a single disciplined regiment that would be required for the use of a Macedonian phalanx formation.

In addition, the later phalanx would never have been useful in the hilly terrain of central Italy. Logic dictates that the Roman soldiers would have trained to fight man to man in a loose formation rather than as large formations of spears from the earliest days of the republic.

Some have argued that the Roman army developed the pilum in response to the Battle of the Allia, which suggests that the Celts had a technological superiority in weaponry. Despite this, the pilum doesn't develop at all like the Celtic javelin. Its dual purpose as a close combat and armor-piercing missile makes it clear that the pilum was a weapon designed solely for war, unlike the Celtic javelin which was used in both hunting and battle.

At Allia, the Roman army was defeated because its barbarian enemy used strategy and planning to overwhelm and overawe the Roman troops. The Senones held their lines and proved to be the more disciplined force, while the Roman morale was shattered. It was not technology or formations but determination that defeated the Roman army, and this defeat changed Rome.



In the aftermath of the Battle of the Allia Brennus led his army into Rome and sacked the city. The elderly were left outside of the citadel and massacred. Women and children were sent away to hide in allied towns along with the religious artifacts and priests. Volunteers and the surviving soldiers holed up on the Capitoline Hill hoping to outlast the Senones.

Famine and disease struck both sides, but in the end, the soldiers that fled to Veii linked up with Latin allies led by Marcus Furius Camillus who cut off the Celtic supply routes and forced the besiegers to abandon the city. Outside of Rome the army caught up to the fleeing Gauls and slaughtered them to the man.

If the Battle of Veii gave Rome a taste of the riches to be found in the wider world, the Battle of the Allia and the sack of Rome taught Rome that the world outside Latium was dangerous. The sack didn't temper Roman pride but stoked and encouraged it. Allia was a disaster because the Roman people had not come together with a single vision.

Patrician and Plebeian would continue to vie for political power, but never again in the Republican era would internal struggles threaten the Republic from an external threat. Rome would ensure that it wasn't threatened again by going on a series of military campaigns through the 4th century that would result in the entirety of the Italian peninsula falling under Roman domination.

Battle of Allia and Sack of Rome Documentary

Further Reading

  • Armstrong, J. (2016). Early Roman warfare: From the regal period to the First Punic War. Barnsley, South Yorkshire: Pen & Sword Military.
  • DeVries, Kelly. Battles That Changed Warfare, 1457 B.C - 1991 A.D.: from Chariot Warfare to Stealth Bombers. New York: Metro Books, 2011.
  • Lendon, J. E. Soldiers & Ghosts: a History of Battle in Classical Antiquity. New Haven: Yale University Press, 2008.
  • Livy, and Betty Radice. Rome and Italy: Books I-V. Harmondsworth, Middlesex: Penguin Books, 1982.
  • Mackay, Christopher S. Ancient Rome: a Military and Political History. Cambridge: Cambridge University Press, 2007.
  • Penrose, Jane. Rome and Her Enemies: an Empire Created and Destroyed by War. Oxford: Osprey, 2005.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 A Anders