The Irish heroic cycles were committed to writing in the Mediaeval period, some time after the pre-Christian era they are supposed to depict. The Welsh Mabinogion dates from roughly the same era. The Táin Bó Cúailnge, chiefly the story of the Ulster hero Cú Chulainn describes individual combats centred on the use of the spear (gae) and javelin (gá-ín) with no mention of helmets or metal armour, in keeping with archaeological evidence. Chariots also play an important role, but without chariot inhumations resembling those of the Britons, no remains of these vehicles from the period have yet been discovered.
Celtic tribes fought amongst each other and sometimes they allied themselves with the Romans, the Greeks and other peoples against other Celtic tribes. Tribal warfare appears to have been a regular feature of Celtic societies. While epic literature depicts this as more of a sport focused on raids and hunting rather than organised territorial conquest, the historical record is more of tribes using warfare to exert political control and harass rivals, for economic advantage, and in some instances to conquer territory.
Archaeology provides much information regarding the material culture of the Celts, but the significance of these finds in determining how the ancient Celts actually fought is the subject of much speculation. It was long thought, for instance, that the Celts were headhunters but recent research from France has indicated that it may have been the heads of slain allies that were collected to be placed in porticos, while the defeated were dumped in mass graves, their weapons ritually broken.
The Hallstatt Culture is the earliest to be identified as associated with Celtic culture, spreading from north of the Alps West into France, Southern Britain and the Iberian Peninsula. The earlier phases of the Hallstatt era fall into the Bronze Age. Swords seem to have been the primary weapon from this period, perhaps indicating that warfare was a relatively small scale affair, possibly between groups of élite warriors. In the latter phases of the Hallstatt era, iron began to replace bronze in the manufacture of weapons and the classic "Celtic longsword" with its leaf-bladed design made its appearance. Chariot burials are also characteristic of the period; it is possible that they also served a function in the warfare of this age, but the chariots are four-wheeled vehicles and they do not occur at all in Britain until the La Tène period. At the very end of the Hallstatt era, the longsword seemed to fall out of favour, ousted by short, thrusting daggers which are found in greater numbers among grave goods in high status burials.
The La Tène Period saw changing patterns of warfare. At the beginning of the La Tène period warfare was likely conducted on a small scale between élite warriors, perhaps in chariots, wielding a new type of Celtic longsword. During the succeeding centuries the design of the sword changed, characteristically becoming shorter, single-edged and lacking a thrusting point, designed purely to make a cut (although the Hallstatt era sword had also been primarily a slashing weapon) and greater regional variation in swords appeared: in Britain and Ireland even the longer sword designs were shorter and thinner than their Continental counterparts. It is possible that in the later La Tène era, an increasing population would have led to larger armies organised in ranks of spearmen, leading to a decline in the importance of the champion with his sword and hence a decline in sword functionality.
The La Tène era also saw the development of armour in the form of mail (chainmail), the familiar form in which a garment is constructed of linked metal rings. Finds of mail are rare, suggesting that it was a luxury restricted to high status warriors. Crested helmets of this period occur in greater numbers than of mail, but overall the picture is of Celtic armies being made up largely of lightly armoured or unarmoured fighters.
Chariot burials continue well into the La Tène period, suggesting their continued importance in warfare. The La Tène chariot was a light, two-wheeled vehicle, unlike the heavier chariot of earlier times. The arrangement of the chariot poles in a reconstruction of the Wetwang Chariot suggests they were drawn by small ponies only 11 or 12 hands high and thus seem unlikely to be used in a frontal charge. Because chariot burials were never practised in Ireland, the nature or existence of chariot warfare in that country remains unclear.
Roman conquest eventually extinguished the independence of all the Celtic peoples except in Ireland and the far North of Britain. After the Roman era, only in the British Isles, therefore, could there be said to still exist a distinctly Celtic style of warfare. Ireland was the last region to adopt the La Tène mode of Celtic technology and with a smaller and less dense population than that of the British or Continental Celts, may have sustained the era of small scale elite combat for longer. Traditional patterns of warfare seem to have continued all the way to the Viking and Norman invasions, conducted by foot soldiers, lacking metal armour including helmets, fighting with spears and javelins, occasionally axes and in the case of higher status warriors, swords, protected by a round or oval shield. The Viking invasions saw the adoption of the bow in addition, but never in great numbers. The Norman invasion in the 12th century and the ineffectiveness of traditional tactics in resisting it led to the Irish moving towards a more typically mediæval style of warfare exemplified by the Gallowglass infantry soldier.
The Gallic Wars were a series of military campaigns waged by the Roman proconsul Julius Caesar against Gallic tribes, lasting from 58 BC to 51 BC. The Romans would also raid Britannia and Germania, but these expeditions never developed into full-scale invasions. The Gallic Wars culminated in the decisive Battle of Alesia in 52 BC, in which a complete Roman victory resulted in the expansion of the Roman Republic over the whole of Gaul. The wars paved the way for Caesar to become the sole ruler of the Roman Republic.
The best known Roman source for descriptions of Celtic warfare is Julius Caesar in his Commentaries on the Gallic Wars (Commentarii de Bello Gallico) in which he describes the methods of warfare of both the Gauls and the Britons.
In describing battles against various tribes of Gauls, in contrast to the popular picture of the wild Celtic berserker, Caesar talks of the Helvetii fighting in close order, forming a phalanx as a defence against cavalry and advancing in a close formation. He also speaks of arrows being used against his troops crossing rivers and against the besiegers of Gergovia, capital of the Arverni - one of the few engagements in which Vercingetorix outmanoeuvred Caesar. He mentions the use of javelins by the Belgic Nervii, but despite Roman writers frequently referring to the use of swords by the Celts in battle, Caesar never mentions Gaulish troops fighting en masse with swords. By the mid-1st century BC, Celtic tribes in Gaul may have had a core of properly trained and equipped Gallic leaders, in addition to the tribal levy of lightly armed and armoured Gauls.
It is perhaps the descriptions of the Britons which have most influenced the popular image of the wild Celtic warrior. Caesar emphasises the "barbarian" aspect of the Britons, possibly for political reasons since his expedition there was of necessity brief, describing how they wore animal skins, had wives in common, did not grow crops and dyed their skin blue: although this description does not mention the plant, subsequent commentators have supposed that woad was the source of this blue dye and though later experimentation suggests that woad is not very well suited as a skin dye nor as tattoo ink, this image, conflated with the descriptions of the Gaesatae, has nevertheless helped paint the picture of the woad-daubed ancient Briton charging into battle naked and blue.
No Celtic group employed a regular military as we would understand it. Organisation was according to clan grouping and social class. A war leader's immediate companions were known in Gaulish as *ambaxtoi ("those who accompany") a term which passed into Latin and from which our own word ambassador ultimately derives.
The earliest encounter with the Romans in 387 BC resulted in all of Rome apart from the Capitoline Hill falling to a confederacy of Gaulish tribes led by Brennus of the Senones. Little or no detail is given of the methods of warfare of these Gauls except that according to Plutarch some were armed with swords and some were mounted. In 280 BC another Brennus led a formidable Celtic army South to attack Greece and Thrace. According to Pausanias this force included large numbers of cavalry, organised in a system called Trimarcisia (from the words *tri- *marko- "three horse") dividing them into teams of three, only two of whom would be mounted at one time. Brennus' expedition would have originated in Pannonia and Noricum, a region which later became famous for producing high quality steel for weaponry. Celts were renowned for their ability to make swords of bronze and iron. Swords were too expensive for many common soldiers so they fought with spears or slings instead.
In earlier times, the Celts would employ the chariot. Although chariots had fallen out of use in continental Europe from the end of the 3rd century BC, Caesar found that they remained significant in British warfare. If his descriptions are to be believed, he encountered in Britain an army in transition, possessing cavalry but still with an elite fighting from chariots. He describes how these warriors would throw javelins from their vehicles before abandoning them to fight on foot and returning to them in order to retreat or redeploy. Cavalry proper is described as used for skirmishing. Gauls are said to have commented that they themselves had formerly used chariots but had abandoned them by this time.
Their mode of fighting with their chariots is this: firstly, they drive about in all directions and throw their weapons and generally break the ranks of the enemy with the very dread of their horses and the noise of their wheels; and when they have worked themselves in between the troops of horse, leap from their chariots and engage on foot. The charioteers in the mean time withdraw a little distance from the battle, and so place themselves with the chariots that, if their masters are overpowered by the number of the enemy, they may have a ready retreat to their own troops. Thus they display in battle the speed of horse, [together with] the firmness of infantry.
The carnyx was a wind instrument of the Iron Age Celts, attested for ca. 300 BC to 200 AD. It is a kind of bronze trumpet, held vertically, the mouth styled in the shape of a boar's head. It was used in warfare, probably to incite troops to battle and intimidate opponents. The instrument's upright carriage allowed its notes to carry over the heads of the participants in battles and ceremonies.
Celtic warriors served as mercenaries in many armies of the classical period. The best known were those who joined Hannibal in his invasion of Italy during the Second Punic war and who contributed to his victories in Lake Trasimene and in Cannae. Celtic mercenaries fought on the sides of Ancient Greeks and Romans as well. When a branch of Brennus' invasion force turned East and crossed the Hellespont, they founded a Celtic-ruled state in Asia Minor known as Galatia. Galatia became well known as a source of mercenaries throughout the Eastern Mediterranean region. Illustrations showing troops armed with long, straight swords and oval shields have generally been taken to depict Galatians.
The Greek historian Polybius gives an account of the Battle of Telamon 225 BC in which the Romans defeated an invasion by the Boii, Insubres, Taurisci and Gaesatae. The Gaesatae were said to be a group of warriors who fought for hire and it is they who are described in the most detail. Whereas the Boii and Insubres wore trousers and cloaks which were thick enough to afford limited protection from the Roman javelins, the Gaesatae removed their clothes to fight naked, standing in front of their allies and seeking to intimidate the Romans with shouting and gesturing. However, this lack of protection caused their defeat since they apparently[weasel words] carried relatively small shields which did not adequately protect them against the missile fire of the Roman skirmishers. Suffering heavy casualties, the Gaesatae either fled the battlefield or desperately charged headlong into the Roman lines where, outmatched for both numbers and equipment, they were defeated. What position the Gaesatae occupied in Celtic society has been much debated. Early writers assumed that they were a tribe, but later authors have inclined to the view that they may have been groups of unattached young warriors who lived by raiding and mercenary activities like the early Roman iuventes or the semi-legendary Irish fiana.
There are accounts of Celtic soldiers in the bodyguards of Cleopatra VII of Egypt and Herod of Judea. Josephus in his Antiquities of the Jews mentions Gallic or Galatian soldiers present at the funeral of King Herod the Great.
While relatively little has been written about Celtic warfare at sea, the Gaulish Veneti, a tribe occupying the South of Brittany fiercely resisted Caesar's Romans both on land and at sea. They were said to have constructed ships of oak with tough leather sails, well adapted for plying the rough Atlantic seas. Their capital, Darioritum, was extremely difficult to attack from land. At first the Roman galleys, fighting in unfamiliar conditions, were at a great disadvantage until new tactics were developed by the Roman admiral Decimus Junius Brutus Albinus which resulted in a decisive victory for Caesar's men. The Veneti were subject to savage reprisals for their defiance.
Large stores of slingstones aerodynamically shaped by adding clay have been found in the Southern British hillfort of Maiden Castle which indicates that slings must also have played a role in the conflicts between Celtic tribes there, probably in sieges. The La Tène period also saw the development of multivallate fortresses, protected by formidable earthworks as well as the Murus Gallicus and Pfostenschlitzmauer constructions. The larger settlements in Gaul were described by Julius Caesar as oppida and the term is now used to designate the large pre-Roman towns that existed all across Western and Central Europe, many of which grew from hill forts. There are over 2000 of these forts known in Britain alone.
The Celtic circular wall of Otzenhausen is one of the biggest fortifications the Celts ever constructed. It was built by Celts of the Treveri tribe, who lived in the region north of the fort. The fort is located on top of the Dollberg, a hill near Otzenhausen in Germany, about 695 m above sea level. The only visible remains are two circular earth ramparts, covered with stones.
Celts were influenced by other peoples but influenced themselves the warfare of their enemies.
The Celts influenced Thracian warfare in the adoption of certain long swords by the Triballi, for example, although this was not universal among the Thracians. Another weapon, the sica, was called the Thracian sword (Ancient Greek: Θρᾳκικὸν ξίφος) though it did not originate there, despite its popular usage. Considered Thrace's national weapon, the sword's ultimate origin was the Hallstatt culture and the Thracians may have or adopted or inherited it.
The Celtic-Germanic Bastarnae were an important part of the Dacian army. Celtic-type weapons were in use, such long swords and round shields. The Celts played a very active role in Dacia and the Celtic Scordisci were among the tribes allied to the Dacians.
Celts affected the Illyrians in cultural and material aspects and some of them were Celticized, especially the tribes in Dalmatia and the Pannonians. A type of wooden oblong shield with an iron boss was introduced to Illyria from the Celts. Hallstatt culture influences abounded as the Illyrians were also its descendants.
The Iberian peninsula, comprising modern Spain and Portugal, was a place of diverse cultures in classical times with various tribes who cannot always be placed firmly as Celts. Iberian Celts (mainly Celtiberians, but also Lusitanians and Cantabrians) fought for Hannibal as mercenaries against the Romans in the Second Punic War. The best known of their weapons was the gladius Hispaniensis, which was adopted by the Romans as their own gladius. The very name, too, may derive from the Celtic root *kledo-, meaning "sword". The Latin word lancea used for the javelins of auxiliary troops, is also supposedly derived from an Iberian or Celtiberian word, but one whose original form is not recorded. The Romans described the spear of the Gauls with the word gaesum, a Latinisation of the Gaulish *gaisos.
It is likely that two Latin words for chariot, carrus and covinnus, were adopted from the Gaulish language, although the Romans at no point seem to have employed chariots in warfare.
Later Greek and early Roman civilization faced major threats from Celtic invaders. Later, the situation was reversed as the expanding Roman Empire gradually conquered most of the Celts. Greek and Roman writers tend to focus much on the savage ferocity of the Celtic warrior, creating an image which has persisted ever since. To the Ancient Greeks and Romans the Celtic warrior was the archetypal barbarian, stereotypically presented as massive, powerful, and malicious. In the 5th century BC a Greek writer Ephoros described the Celts as one of the four great barbarian peoples, along with the Persians, the Scythians and the Libyans. They were called Keltoi or Galatae by the Greeks and Celtae or Galli by the Romans. Aristotle comments that their courage had an element of passion like that of all barbarians. Diodorus Siculus writes that they were extremely addicted to wine and that one could exchange a mere jar of wine for a slave.
The Celts were described by classical writers such as Strabo, Livy, Pausanias, and Florus as fighting like "wild beasts", and as hordes. Dionysius said that their "manner of fighting, being in large measure that of wild beasts and frenzied, was an erratic procedure, quite lacking in military science. Thus, at one moment they would raise their swords aloft and smite after the manner of wild boars, throwing the whole weight of their bodies into the blow like hewers of wood or men digging with mattocks, and again they would deliver crosswise blows aimed at no target, as if they intended to cut to pieces the entire bodies of their adversaries, protective armour and all". Such descriptions have been challenged by contemporary historians.
This is a list of battles or conflicts in which Celts had a leading or crucial role, including as mercenaries.