
Gallic groups, originating from the variousLa Tène chiefdoms, began a southeastern movement into theBalkans from the 4th century BC. Although Gallic settlements were concentrated in the western half of theCarpathian basin, there were notable incursions and settlements within the Balkans.
From their new bases in northernIllyria andPannonia, the Gallic invasions climaxed in the early 3rd century BC, with the invasion ofGreece. The 279 BC invasion ofGreece proper was preceded by a series of other military campaigns waged in the southern Balkans and against theKingdom of Macedonia, favoured by the state of confusion ensuing from thedisputed succession after Alexander the Great's death. A part of the invadingCelts crossed over toAnatolia andeventually settled in the area that came to be named after them,Galatia.
From the 4th century BC,Celtic groups pushed into theCarpathian region and theDanube basin, coinciding with their movement intoItaly. TheBoii andVolcae were two large Celtic confederacies who generally cooperated in their campaigns. Splinter groups moved south via two major routes: one following theDanube, another eastward from Italy. According to legend, 300,000 Celts moved into Italy andIllyria.[1]
By the 3rd century, the native inhabitants ofPannonia were almost completelyCelticized.[2]La Tène remains are found widely in Pannonia, but finds westward beyond theTisza and south beyond theSava are rather sparse.[2] These finds are deemed to have been locally producedNorican-Pannonian variation ofCeltic culture. Nevertheless, features are encountered that suggest ongoing contacts with distant provinces such asIberia. The fertile lands around the Pannonian rivers enabled the Celts to establish themselves easily, developing their agriculture and pottery, and at the same time exploiting the rich mines of modernPoland. Thus, it appears that the Celts had created a new homeland for themselves in the southern part ofCentral Europe; in a region stretching from Poland to the Danube, but there is little to no non-Christian evidence of that.[clarification needed]
The political situation in the northern Balkans was in constant flux with various tribes dominant over their neighbours at any one time. Within tribes, military expeditions were conducted by "an enterprising and mobile warrior class able from time to time to conquer large areas and to exploit their population".[2] The political situation in the Balkans during the 4th century BC played to the Celts' advantage. The Illyrians had been waging war against the Greeks, leaving their western flank weak. WhileAlexander ruled Greece, the Celts dared not to push south near Greece. Therefore, early Celtic expeditions were concentrated against Illyrian tribes.[3]
The first Balkan tribe to be defeated by the Celts was theIllyricAutariatae, who, during the 4th century BC, had enjoyed a hegemony over much of the central Balkans, centred on theMorava valley.[2] An account of Celtic tactics is revealed in their attacks on theArdiaei.[further explanation needed]
In 335 BC, the Celts sent representatives to pay homage toAlexander the Great, whileMacedon was engaged in wars againstThracians on its northern border. Some historians suggest that this 'diplomatic' act was actually an evaluation of Macedonian military might.[3] After the death of Alexander the Great, Celtic armies began to bear down on the southern regions, threatening the kingdom of Macedonia and the rest of Greece. In 310 BC, the Celtic general Molistomos attacked deep into Illyrian territory, trying to subdueDardanians,Paeonians andTriballi. However Molistomos was defeated by the Dardanians. The new Macedonian kingCassander felt compelled to take some of his old Illyrian enemies under his protection even though the Illyrians emerged victorious.[3] In 298 BC, the Celts attempted a penetrating attack intoThrace andMacedon, where they suffered a heavy defeat nearHaemus Mons at the hands of Cassander. However, another body of Celts led by the general Cambaules marched on Thrace, capturing large areas.[1] TheCeltic tribe of theSerdi[4] lived inThrace and founded the city ofSerdica, present daySofia.

The Celtic military pressure toward Greece in the southern Balkans reached its turning point in 281 BC. The collapse ofLysimachus' successor kingdom in Thrace opened the way for the migration.[5] The cause for this is explained byPausanias as greed for loot,[6] byJustin as a result of overpopulation,[7] and byMemnon as the result of famine.[8] According toPausanias, an initial probing raid led by Cambaules withdrew when they realized they were too few in numbers.[6] In 280 BC, a great army comprising about 85,000 warriors[9] leftPannonia, split into three divisions, and marched south in a great expedition[10][11] to Macedon andcentral Greece. Under the leadership ofCerethrius, 20,000 men moved against the Thracians and Triballi. Another division, led byBrennus[12] andAcichorius[13][14] moved against thePaionians, while the third division, headed byBolgios, aimed for theMacedonians andIllyrians.[6]

Bolgios inflicted heavy losses on the Macedonians, whose young king,Ptolemy Keraunos, was captured and decapitated.[15][16] However, Bolgios' contingent was repulsed by the Macedonian noblemanSosthenes, and satisfied with the loot they had won, Bolgios' contingents turned back. Sosthenes, in turn, was attacked and defeated by Brennus and his division, who were then free to ravage the country.
After these expeditions returned home, Brennus urged and persuaded them to mount a third united expedition against central Greece, led by himself and Acichorius.[6] The reported strength of the army of 152,000 infantry and 24,400 cavalry is impossibly large.[17] The actual number of horsemen has to be intended half as big:Pausanias describes how they used a tactic calledtrimarcisia, where each cavalryman was supported by two mounted servants, who could supply him with a spare horse should he have to be dismounted, or take his place in the battle, should he be killed or wounded.[18][19]
A Greek coalition made up ofAetolians,Boeotians,Athenians,Phocians, and other Greeks north ofCorinth took up quarters at the narrow pass ofThermopylae, on the east coast of centralGreece. During the initial assault, Brennus' forces suffered heavy losses. Hence he decided to send a large force under Acichorius againstAetolia. The Aetolian detachment, as Brennus hoped, left Thermopylae to defend their homes. The Aetolians joined the defenceen masse – the old and women joining the fight.[20] Realizing that the Gallic sword was dangerous only at close quarters, the Aetolians resorted toskirmishing tactics.[5] According to Pausanias, only half the number that had set out for Aetolia returned.[6]
Eventually, Brennus found a way around the pass at Thermopylae, but by then the Greeks had escaped by sea.

Brennus pushed on toDelphi, where he was defeated and forced to retreat, after which he died of wounds sustained in the battle. His army fell back to the riverSpercheios, where it was routed by theThessalians andMalians.
Both historians who relate the attack on Delphi (then member of theAetolian League), Pausanias andJunianus Justinus, say that the Gauls were defeated and driven off. They were overtaken by a violent thunderstorm, which made it impossible to manoeuvre or even hear their orders. The night that followed was frosty, and in the morning the Greeks attacked them from both sides. Brennus was wounded and the Gauls fell back, killing those of their own wounded who were unable to retreat. That night, a panic fell on the camp, as the Gauls divided into factions and fought amongst themselves. They were joined by Acichorius and the rest of the army, but the Greeks forced them into a full-scale retreat. Brennus took his own life by drinking neat wine[clarification needed] according to Pausanias, or by stabbing himself according to Justinus. Pressed by the Aetolians, the Gauls fell back to the Spercheios, where the waiting Thessalians and Malians destroyed them.[18][21]
In spite of the Greek accounts about the defeat of the Gauls, theRoman literary tradition preferred a far different version.[clarification needed]Strabo reports a story told in his time of a semi-legendary treasure – theaurum Tolosanum, fifteen thousand talents (450 metrictonnes/990,000 pounds) of gold and silver – supposed to have been the cursed gold looted during the sack of Delphi and brought back to Tolosa (modernToulouse,France) by theTectosages, who were said to have been part of the invading army.
More than a century and a half after the alleged sack, the Romans ruledGallia Narbonensis. In 105 BC, while marching toArausio, theProconsul ofCisalpine GaulQuintus Servilius Caepio plundered the sanctuaries of the town ofTolosa, whose inhabitants had joined theCimbri, finding over 50,000 15 lb. bars of gold and 10,000 15 lb. bars of silver. The riches of Tolosa were shipped back to Rome, but only the silver made it: the gold was stolen by a band of marauders, who were believed to have been hired by Caepio himself and to have killed the legion guarding it. The Gold of Tolosa was never found, and was said to have been passed all the way down to the last heir of the Servilii Caepiones,Marcus Junius Brutus.
In 105 BC, Caepio refused to co-operate with his superior officer,Gnaeus Mallius Maximus, because he thought of him as anovus homo, deciding by himself to engage in battle against theCimbri, on theRhone. There, the Roman army suffered a crushing defeat and complete destruction, in the so-calledBattle of Arausio (modernOrange).
Upon his return to Rome, Caepio was tried for "the loss of his Army" andembezzlement. He was convicted and given the harshest sentence allowable; he was stripped of hisRoman citizenship, forbidden fire and water within 800 miles of Rome, fined 15,000talents (about 825,000 lb) of gold, and forbidden from seeing or speaking to his friends or family until he had left forexile.
He spent the rest of his life in exile inSmyrna inAsia Minor. His defeat and ensuing ruin were looked upon as a punishment for his sacrilegious theft.
Strabo distances himself from this account, arguing that the defeated Gauls were in no position to carry off such spoils, and that, in any case, Delphi had already been despoiled of its treasure by thePhocians during theThird Sacred War in the previous century.[22] However, Brennus' legendary pillage of Delphi is presented as fact by some popular modern historians.[23]

Most scholars deem the Greek campaign a disaster for the Celts.
Some of the survivors of the Greek campaign, led by Comontoris (one of Brennus' generals) settled inThrace. In 277 BC,Antigonus II Gonatas defeated the Gauls at theBattle of Lysimachia and the survivors retreated, founding a short-lived city-state namedTyle.[24] Another group of Gauls, who split off from Brennus' army in 281 BC, were transported over toAsia Minor byNicomedes I to help him defeat his brother and secure the throne ofBithynia. They eventually settled in the region that came to be named after them,Galatia. They were defeated byAntiochus I, and as a result, they were confined to barren highlands in the centre of Anatolia.[25]
Celtic groups were still the pre-eminent political units in the northern Balkans from the 4th to the 1st century BC. TheBoii controlled most of northern Pannonia during the 2nd century BC, and are also mentioned as having occupied the territory of modernSlovakia. We learn of other tribes of the Boian confederation inhabiting Pannonia. There were theTaurisci in the upper Sava valley, west ofSisak, as well as theAnarti,Osi andCotini in theCarpathian basin. In the lowerSava valley, theScordisci wielded much power over their neighbours for over a century.
The later half of the 1st century BC brought much change to the power relations of barbarian tribes in Pannonia. The defeat of the Boian confederation by theGeto-Dacian kingBurebista significantly curtailed Celtic control of the Carpathian basin, and some of the Celticization was reversed. However, more Celtic tribes appear in sources. TheHercuniates andLatobici migrated from the northern regions (Germania). Altogether new tribes are encountered, bearing Latin names (such as theArabiates), possibly representing new creations carved out of the defeated Boian confederation. To further weaken Celtic hegemony in Pannonia, the Romans moved the Pannonian-IllyrianAzali to northern Pannonia. The political dominance previously enjoyed by the Celts was overshadowed by newer barbarian confederations, such theMarcomanni andIazyges. Their ethnic independence was gradually lost as they were absorbed by the surrounding Dacian, Illyrian and Germanic peoples, although Celtic names survive until the 3rd century AD.[26]
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