The evolution of the Celtic Empire is very complex. When we are dealing with the classical sources, there is sometimes confusion with the various names that were used. The Greeks called the Celts, "Keltoi" and "Galatae". The Romans also used two names to describe the Celtic tribes, "Celtae" and "Galli".
By the sixth century BC the Celtic Empire stretched from France and I imagine in Britain and Ireland, through to Spain as far as Cadiz and through to the Po Valley of Northern Italy. In fact the Celts dominated Northern Italy after defeating the Etruscan empire and Rome in Circa 390-387 BC. In fact the Po plain and the adjacent areas were under Celtic rule and called Gallia Cisalpina translating as "Gaul this side of the Alps." Here is Diodous Siculus describing the Celtic lands:
"At the time that Dionysius was besieging Rhegium (391 BC), the Celts who had their homes in the regions beyond the Alps streamed through the passes in great strength and seized the territory that lay between the Alpennine Mountains and the Alps, expelling the Tyrrhennians who lived there."
When the Celts attacked the Etruscans, in 390-387 BC, the Etruscans appealed to Rome for help against the invaders, the Celtic tribe of Senones. Rome was now sacked by the Gauls, who totally devastated the city and forced the Romans to pay a ransom for the release of their city. It is said that when the Romans complained about how the ransom was being weighed out, the head of the Gallic army, Brennus threw his sword on the scales and cried out "vae victus!"which translates as "woe to the conquered". I think that is still apt today. After this the Gauls were known as "terror Gallicus" to the Romans, and greatly feared.
The Celts met Alexander the great for a peace conference in 335 BC. Here is the meeting described by Strabo:
"The Celti who lived about the Adriatic joined Alexander for the sake of establishing friendship and hospitality... the king received them kindly and asked them when drinking what it was that they most feared, thinking they would say himself, but they replied that they feared nothing, unless it were that Heaven might fall on them, although indeed they added that they put above everything else the friendship of such a man as he."
In 281 BC the Celts attacked Macedonia, defeating them and beheading their king, King Ptolemy Ceraunus. In 279 BC, the Celts under their leader, Bolgios, invaded Macedonia. Now the victorious Celtic army split in two, there was a dispute. 20,000 men led by Leotarios and Leonnorious went to conquer Turkey, and founded the Celtic State of Galatia there. Another part of the Celtic army, and it must have been huge went under the leaders Acichorois and Brennus to conquer Greece.
I will first deal with the Celtic attack on Greece. Here is Pausanias, describing the Celtic forces and Brennus deciding to attack Greece:
"It was then that Brennus, both in public meetings and also in personal talks with individual Galatian officers, strongly urged a campaign against Greece, enlarging on the weakness of Greece at the time, on the wealth of the Greek states, and on the even greater wealth in sanctuaries, including votive offerings and coined silver and gold. So he induced the Galactians to march against Greece... The muster of foot amounted to one hundred and fifty-two thousand, with twenty thousand four hundred horse..."
So in 279 BC, the Celts marched into Greece. There intention to sack and wreck one of the most famous holy places in the ancient world, the Temple of Apollo at Delphi. The Greeks were horror struck, many tales of Celtic cruelty had reached them. The Celts by this time had a reputation for atrocities, although they were likely to be propaganda to scare the enemy, they were greatly feared. Here is a horrific description of the Celts at war given by Pausanias in his book the Description of Greece:
"The fate of the Callians at the hands of (the Galatians or Celts) is the most wicked ever heard of, and is without parallel in the crimes of men. Every male they put to the sword, and there was butchered old men equally with children at their mother's breast. The more plump of these sucking babes the Gauls killed, drinking their blood and eating their flesh."
Don't they sound ferocious, really evil, no wonder the Greeks were scared. Talk about demoralising the enemy.
The Galatians reached Delphi, to attack the Temple of Apollo in mid winter. The great battle began. The Greeks asked the gods for help to protect their sacred temple which was the focus point of their lives. They pleas were answered and there were earthquakes and thunderbolts and even rock slides from nearby Mount Parnassus on the enemy. Still the Celts fought on. The Greeks again asked for divine help. During the night, the Celts were said to panic and fight each other. Pausanias described the mayhem as "causeless terrors are said to come from the god Pan". Eventually the Celts retreated after suffering grievous losses, 26,000 dead, according to the Greek historian Pausanias. Here is Pausanias describing the battle which was fought with divine aid:
"Brennus and his army were now faced by the Greeks who had mustered at delphi, and soon portents boding no good to the barbarians were sent by the god... the whole ground occupied by the Galatian army was shaken violently ... with continuous thunder and lightening ... the thunder terrified the Galatians and prevented them from hearing their orders, while the bolts from heaven set on fire not only those whom they struck but also their neighbours."
Isn't that a wonderful description. Whether the atrocious weather conditions were the result of the mid winter or the pleas to the Greek gods, I will leave to your imagination. But it is a wonderful example of the religious thinking of the period, when divine intervention was thought to save one of the most important religious sites in the then known world.
After the defeat, Brennus died, it is said that he took his own life by drinking undiluted wine. This battle marked the high point to the Celtic invasions of Europe. In 278-277 BC, the king of Macedonia, King Antigonos Gonatas defeated the Celtic army at Lysimachia. The Celtic invasion of Greece was over and their influence in Europe now gradually lessened.