Battle of the Allia The Battle of the Allia was a battle of the first Gallic invasion of Rome . The battle was fought near the Allia river : the defeat of the Roman army opened the route for the Gauls to sack Rome . It was fought in 390 / 387 BC . Background . Prior to the battle , the Senones , a single tribe of Gaul , traversed the Appennines searching for new land to settle . They eventually camped outside the town of Clusium ( in the Etruscan province of Siena ) and began negotiations for land rights . The Clusians felt threatened by the Senones , and they called for help from Rome , who had recently exerted military influence over Etruria . Rome , weakened by recent wars , sent a delegation of three ambassadors , the Fabii brothers , to negotiate the situation . When negotiations broke down , the Clusians sent an army to force the Senones off the land . At this point , the Roman historian Livy stated that the Roman ambassadors " broke the law of nations " ( that is , broke their oath of neutrality as ambassadors ) " and took up arms " against the Senones . In the resulting action , Quintus Fabius , an ambassador and a member of a powerful patrician family , killed one of the Gallic leaders ( a chieftain ) . When the Senones realized that the sacred trust of the ambassador was broken , they withdrew from battle to discuss the issue . The Senones sent their own ambassadors to Rome , demanding the Fabians be handed over to them for justice . Many Romans ( especially priests ) were sympathetic , and agreed that it was a breach of the law of nations . However , the Roman masses mocked the priests , and as Livy writes , " those who ought to have been punished were instead appointed for the coming year military tribunes with consular powers ( the highest that could be granted ) . . . The Celtic ( Gallic ) envoys were naturally - and rightly - indignant ! " The enraged Senones promised war against the Romans to avenge the insult that they had been dealt . The Senones marched 130 km from Clusium to Rome to take revenge . Livy describes their journey : " Contrary to all expectation the Celts ( Gauls ) did them [ the people of the countryside ] no harm , nor took aught from their fields , but even as they passed close by their cities , shouted out that they were marching on Rome and had declared war only on the Romans , but the rest of the people they regarded as friends . " Thus , the Senones came to engage the Roman army about 18 km north of the city , at the Battle of the Allia . Roman disaster . According to the common ( but incorrect ) Varronian chronology , the battle took place on July 18 , 390 BC , but a more plausible date is 387 . About 24 , 000 Romans under Quintus Sulpicius fought against the Senones , a Gallic tribe who were about half that number , under Brennus . The Romans , with six legion s , took post on the Allia to check the advance of the Senones on Rome . In those days , a legion counted 4 , 200 men , but was rarely fully manned . The Roman army was at this time a militia and very similar to a Greek phalanx battle line , with heavy hoplite s in the centre ( representing the richer Roman citizens ) and extending to flanks with poorer and poorly armed conscripts ( every soldier was required to supply his own equipment ) . When the Gauls attacked , the Roman flanks were routed leaving the Roman centre to be surrounded and slaughtered . Many of Rome ' s older citizens made up this centre and they would be sorely missed in the coming calamity . While the left wing fled to Veii , the survivors of the right wing fled back to Rome in panic ; as Livy states , " all hastened to Rome and took refuge in the Capitol without closing the gates . " In Rome the citizens barricaded themselves on the Capitoline Hill . The Gauls unsuccessfully tried a full frontal assault , pausing half way up the hill and due to the Romans ' quick thinking , and a direct charge later , they paid dearly and lost many lives . At this point , the besieged Roman soldiers in Veii needed to get a message to the Senate in order to reinstate Marcus Furius Camillus as the dictator and general , and so a messenger climbed a steep cliff the Gauls had neglected to guard . The messenger left with the Senate ' s approval , but the Gauls noticed this path onto the hill . According to legend Marcus Manlius Capitolinus was alerted to the Gallic attack by the sacred geese of Juno . The rest of the city was plundered and almost all Roman records were destroyed . Marcus Furius Camillus may have arrived with a relief army , but this may be Roman propaganda to help quell the humiliation of defeat . The Gauls may have been ill - prepared for the siege , as an epidemic broke out among them as a result of not burying the dead . Brennus and the Romans negotiated an end to the siege when the Romans agreed to pay one thousand pounds of gold . According to tradition , to add insult to injury , it was discovered that Brennus was using heavier weights than standard for weighing the gold . When the Romans complained , Brennus is said to have thrown his sword and belt on the scales and adding in Latin , " Woe to the vanquished " ( " vae victis " ) , in conclusion ( Livy V . 48 ) . According to some Roman historians , it was in this very moment that Camillus arrived with a Roman army and , after putting his sword on the scale , replied , " Not gold , but steel redeems the native land , " thus attacking the Gauls . A battle ensued in the streets of Rome , but neither army could fight effectively in the narrow streets and alleyways . The Gallic and Roman armies left the city and fought the next day . Camillus ' army lived up to his hopes and the Gallic army was routed . The Romans dubbed Camillus a " second Romulus , " a second founder of Rome . The entirely unreliable " Historia Regum Britanniae " , a medieval work written c . 1136 by Geoffrey of Monmouth on the legendary kings of Britain , states that Brennius led both Britons and Gauls . He besieged Rome for three days until his brother came to aid in the invasion . The Romans defended the city for many days and were successful in repelling the invaders . Finally , the two consuls put on armour and joined the men defending the city . They pushed the invaders back but Belinus ( Brennius ' brother ) was able to reform the lines and stop the attacks . Brennius and Belinus continued forward until the walls were breached and the Britons and Gauls invaded the city . According to this story Brennius stayed in Rome and ruled ruthlessly for the rest of his days , although this seems highly unlikely . Recovery and reform . It was considered that the capital be moved to Veii , but the Senate decided against it when a centurion was overheard saying to his men " Let us stay here . " It is conjectured that there was no effective wall around Rome before the siege because Rome ' s earlier Etruscan rulers may have forced the Romans to dismantle significant defences . As a result of the siege and near total destruction of Rome , Rome built the much stronger Servian Wall . The Romans also began restructuring their military organization : They ceased using the Greek phalanx style spear and adopted better and more standardized armour and weapons . The massacre of the 1st Class infantry , representing many Patrician and aristocratic citizens , enforced the need not to expose such important members of Roman society . It could have been at this time that the Triarii ( remnants of the 1st Class ) were formed , as a reserve . Many historians speculate that the Romans learned much about weapons technology and battle tactics from this run - in with the Senones . Though only a single tribe , the Senones were part of the much larger culture of Celts ( or Gauls ) that had more advanced iron - working and close - quarter combat techniques . Specifically , the Celts / Gauls used heavier long swords and full body shields , which allowed them to interlock shields for greater defense ( a tactic later named " tortoise " ( " testudo " ) in the Roman histories ) . Later , after Roman defeats in the second Samnite War had shown novel enemy tactics and formations , it was recognized the need for increased flexibility , leading to the reorganization of the legion into three main lines of soldiers : the hastati in front , the principes in the middle , and the triarii in the rear organized in alternating " maniples " ( units ) . This was to be known as " manipular formation " . Lightly equipped men who had been fighting in the legion for up to two years would fight in the Velites rank in the far front , throwing javelins at the enemy and then retreating . Men with more experience would fight in the next two ranks armed with a heavy javelin , a short sword and a shield : Hastati in the front , veteran Principes behind them . Finally the older Triarii would be in the rear , organized in smaller units of 60 men as opposed to 120 in the front ranks . The Triarii were armed in Hoplite weapons and armor . The Romans had created a " teaching army " that would introduce the young Velites to battle while minimizing the chances of death . The military system that resulted remained the basis of all Roman armies for the next few centuries , as well as the instrument that made possible the Roman Empire . The defeat at the hands of the Gauls was the last time the city of Rome was captured by non - Roman forces until 410 AD .