A great battle indeed, one the Romans never forgot. Polybius dates it to 387, or, as he says, the 19th year after the battle of Aegospotami. But later Roman annalists culminating with Varro dated it in 390 BC or the year 364 of the founding of the City. So this date is given in many references, including Mommsen who devotes several pages to this battle and the subsequent sack of Rome in Book I.

While Polybius mentioned the battle by way of fixing the beginning of his history, he did not describe it. For a description, one has to read Titus Livy. The battle and sack is also described in a convoluted way by Plutarch in his Life of Camillus - see any edition of Plutarch's Lives of the Greeks and Romans. This part of the history of Rome is also contained in Diodorus Siculus' history. Recent authors one can read more readily are :

Herm, Gerhard, The Celts, St. Martin's Press, New York, 1975. He gives a good account of the battle. Incidently the Romans called these people Galli and the Greeks called them Galatai or Keltoi. Thus you have the same people invading Greece and also settling into western Asia Minor (Galatia), northern Italy (Cisalpine Gaul) and most of France and parts of Ireland, Spain, etc. They were the Halstadt Culture people from the Danube valley, tall and powerful warriors who disdained to wear armor, but stripped for battle. They were formed into a large number of tribes. It was mainly the Senones who captured Rome in 387.

Powell, T. G. The Celts, 1958. and Hubert, H. The Rise of the Celts, 1934 are earlier standard works. Two books that discuss the battle from the Roman historical perspective are:

Scullard, H. H. A History of the Roman World 753 - 146 BC New York, Barnes and Noble, 1961. and

Dudley, Donald R. The Romans 850 BC - 337 AD, New York, Alfred Knopf, 1970.

Here is a brief background on the situation and events.

Rome was in midst of its early expansion with wars against its neighbors, notably the Etruscans in the 5th century BC. At the end of the century it was engaged in a bitter struggle with Veii, a strongly fortified Etruscan city. The great Roman hero and dictator, Camillus, conducted a long siege that ended in 396 with Rome destroying Veii and annexing all its territory.

Thus it was that the sudden arrival of the Gauli (Celts) was such a stunning blow. The Celts had already crossed the Alps and pushed the Etruscans out of most of the Po River valley. These Celts were a small part of the general migration of tribes from their empire's base along the Danube. The Insubres were among the first to defeat the Etruscans near Melpum (Milan). They were followed by the Cenomani, Boii, and Lingones. Then, in 390 one of the late arriving Celtic tribes, the Senones, crossed the Apennines and appeared before Clusium.

They were looking for plunder, rather than for land to settle. The town apparently sought help from Rome. Roman envoys arrived to negotiate, but instead of acting as neutrals they joined the locals and one even may have killed a Celtic chief. The Romans are said to have insulted the Celts, who promptly marched on the city. (This part of the story may be a legend designed to explain the subsequent disaster in terms of retribution or divine punishment). In any event the Celts abandoned the siege of Clusium and decided on seeking the greater potential of plunder at Rome itself.

The Celtic army numbered perhaps 30,000 savage warriors. The Roman army was hastily drawn up in battle by the river Allia, a tributary of the Tiber. They had only two legions, which would be about 10,000 counting cavalry detachments, still the largest army Rome had fielded by that time..(Some ancient sources like Diodorus give the strength as 70,000 Gauls versus 40,000 Romans, but they usually exaggerate).

According to the legend, they were abandoned by all their allies. They had never faced such warriors before. The army was drawn up for battle about 11 miles from the city, near Fidenae. The various authorities disagree on which bank of the Tiber they stood. Mommsen says the right bank, but Scullard argues well for the left bank. The Celts quickly turned the Roman flank and drove most of them into the river. Others fled in panic to Veii, leaving the way to the city open. Livy repeats the various legends that grew up about the subsequent events.

This was one of, if not the most, traumatic event in the city's early history. When the Celtic army, led by Brennus arrived they found the city gates open and the streets apparently deserted. The Celts arrived 3 days after the battle, by which time the priests and Vestal Virgins had managed to carry off much sacred regalia to Caere. The legend says the Celts found only elderly Senators seated on their official seats and were temporarily in awe. Soon, however, they recovered their composure and killed the Senators and sacked the city. Only the fortified Capitol held out. Here the sacred geese aroused M. Manlius Capitolinus in time to ward off a Celtic surprise attack.

Meanwhile, the survivors of the Allia at Veii begged Camillus to return from exile and accept the dictatorship. The Celts besieged the citadel for seven months. The starving Romans finally bought them off with a tribute of a thousand pounds of gold. According to one legend Camillus arrived with a relief army and drove the Celts away. According to another famous story, during the counting out of the tribute the Romans accused the Celts of using faulty weights. At this Brennus is said to have thrown his sword onto the scale calling out "vae victis" (woe to the vanquished), in other words you have to add that much more gold just for complaining.

This is the subject of famous paintings throughout history. The many legends are designed to save Rome's honor, but it is clear that the Celts never intended to stay, but only to plunder, and left when they were good and ready.

The results of the destruction are still found in the archaeological explorations of the Palatine hill. Rome had to spend the next century again rebuilding not only the city, but with more difficulty its power and control over central Italy. On the positive side, long-lasting fear of the Celts helped bring the many Italian cities together and helped Rome on its path to unify the region.

Rome's resolve in rebuilding and unifying the Italian cities resulted in the Celts being finally driven into France, otherwise much of Italy would have become Celtic land. Among the measures promptly taken was the fortification of the entire city. Camillus is credited with the heroic leadership in this effort. The earlier earthen wall and wooden palisade was replaced with a stone wall at least 12 feet thick and 24 feet high in front of the earthen wall of the same dimensions. This Servian wall stretched 5.5 miles, making Rome a mighty fortress. Most likely Greek engineers were hired for the technical design and planning and the Roman army provided the labor.

Another result of the disastrous battle and sack was that the need for a larger army was apparent and that meant extending service, and hence citizenship, to more people. The next century was full of internal struggle over the rights of the newly inducted lower classes.

On the foreign side, the Romans had to start over with warfare against their neighbors on all sides. Not only Etruscans, but also Hernici, equi, Volsci and Gauls (Celts). For this they had to give more rights to the other Latins.

During the first decades after they sacked the city the Celts were content to remain in the Po River region, except for occasional raids. But many served as mercenaries in various armies throughout Italy. In 332-331 the Celtic campaign from the Danube toward Greece was blocked temporarily by Alexander the Great.

One of the greatest of the Roman wars with the Celts came in 285 BC and in this one the Romans were victorious. The Celts did the Romans an indirect favor when Italy was invaded in 280 BC by Pyrrhus. As the king of Epirus was engaging in indecisive engagements with the Romans, the Celts invaded Macedonia. They returned to the attack against Rome with a vengeance during the Punic Wars and provided some of the best of Hannibal's troops. In fact, the Romans feared them more than they did the Carthaginians. In 225 the Celts launched yet another offensive against Rome, only to be soundly defeated despite their incredible valor in combat.

I have discussed these events at length in order to show that, although the events on the Allia battlefield were over rather quickly and are not discussed in detail in the ancient sources, the results of the battle were felt for generations. And, thanks to the legends, repeated by Livy and others, the battle remained vivid even for Renaissance artists and authors.