Not a nation, and not exactly forgotten
The Celts were never a single nation, empire or political unit, and they are not forgotten in the sense of being unknown. Greek and Roman authors wrote about them for centuries, and their archaeological remains stretch from Spain to Turkey. What is true is that they left almost no writing of their own, so their history is mostly reconstructed through the eyes of outsiders and through excavation. The word Celtic today has two main uses. Linguists use it for a family of related languages. Archaeologists and historians use it, with more care, for Iron Age societies of temperate Europe that shared styles of art, dress and weaponry and, in many cases, languages in this family.
How the name arose
The earliest known mentions are Greek. Hecataeus of Miletus, around 500 BCE, refers to Keltoi in the interior of western Europe, and Herodotus places them near the source of the Danube. Julius Caesar later wrote that the people of central Gaul called themselves Celtae in their own language, while the Romans called them Galli. The Greeks used Galatai as a related name. Writers of the time did not generally call the peoples of Britain or Ireland Celts. That usage grew up in the early modern period, when scholars such as Edward Lhuyd recognised in the early 1700s that Irish, Welsh, Breton and the vanished language of Gaul belonged to one family. Their descendants adopted the label for themselves.
Archaeological roots
Archaeologists link the early Celtic world to two phases of the central European Iron Age. The Hallstatt culture, from roughly 800 to 450 BCE, is named after a cemetery near salt mines in Austria, and Hallstatt salt mining was an important source of wealth. Fortified hilltop settlements such as the Heuneburg on the upper Danube and rich graves with Mediterranean imports belong to its later centuries. The La Tène culture followed from about the mid-fifth century BCE and is known for its art and for large fortified towns, or oppida, in the last two centuries BCE.
Trade ran through both periods. The amber trade routes carried Baltic amber south, and wine and metal vessels came north from Greek and Etruscan sources. Languages cannot be read from archaeology, though, and not everyone who used La Tène style objects need have spoken a Celtic language.
Expansion into southern Europe and Anatolia
From the fourth century BCE, groups usually called Gauls or Celts moved south and east. The Roman tradition places a Gallic sack of Rome in about 390 BCE, though dates in ancient chronology vary. In 279 BCE a Celtic force raided northern Greece and threatened the Delphi sanctuary, and soon after, groups crossed into Asia Minor, where they settled in the central plateau as the Galatians. The Hellenistic kings of Pergamon celebrated victories over them with monumental sculpture. The later Pergamon Altar belongs to the same city’s tradition of victory monuments, though its main frieze shows gods and giants rather than Gauls. Writers of late antiquity suggest that a Celtic language was still spoken in Galatia centuries later.
Celtic-speaking communities also settled in northern Italy, which the Romans called Cisalpine Gaul, until Rome overcame them over the third and second centuries BCE. During the Punic Wars, Celtic warriors fought on both sides, and Hannibal recruited many in northern Italy.
Gaul
Gaul is the best-documented region, thanks mostly to Julius Caesar. He divided it into three parts, including the Belgae of the north and the Celtae of the centre, and he named dozens of peoples. Among the most powerful were the Aedui, the Arverni, the Sequani, the Helvetii and the Treveri. They had chiefs or elected magistrates, assemblies, clients and rivals. Many struck their own coins and lived in oppida such as Bibracte, the Aeduan stronghold. Rivalries among them helped Caesar during the conquest of Gaul between 58 and 50 BCE, which ended in 52 BCE at Alesia, when the Arvernian leader Vercingetorix surrendered to Rome. The campaign relied on Roman legions with allies, and it was recorded by Caesar himself, who had every reason to present it favourably.
Britain
In Britain, Caesar’s brief expeditions of 55 and 54 BCE were followed by Roman conquest from 43 CE. Roman authors and coins name peoples such as the Catuvellauni, the Trinovantes, the Iceni, the Brigantes, the Silures and the Durotriges. They lived in farmsteads, hillforts and, in the south-east, early towns with coin-issuing rulers. The Iceni’s revolt under Boudica in 60 or 61 CE is the best-known episode.
Whether Britons were “Celts” is a matter of debate. They spoke related Brittonic languages, from which Welsh, Cornish and Breton descend (Breton arrived with migrants in late antiquity), and they shared artistic styles, but the label comes from later scholars. Northern Britain beyond the Roman frontier was home to peoples later known as Picts, and relations with the Picts and Romans form their own story.
Ireland
Ireland was never conquered by Rome. Our knowledge of its Iron Age is therefore thin, based on archaeology such as ritual sites and artefacts, and on a map of tribal names preserved in the Geography of Ptolemy in the second century CE. Early Irish writing began only with Ogham inscriptions, from perhaps the fourth or fifth century CE and then with Latin-alphabet texts after Christianisation. The great Irish sagas were written down by monks in the early Middle Ages, and while they preserve older material, they cannot be treated as accounts of the Iron Age.
Iberia
In Iberia, the Celtiberians of the central plateau are known from Roman accounts and from inscriptions in their own language, Celtiberian, written in a local script derived from the Iberian one. The siege and fall of Numantia in 133 BCE is a famous episode of their resistance. Peoples in the north-west, such as the Gallaeci, are often called Celtic, but how far they spoke Celtic languages is debated.
Society and religion
Celtic societies were typically ruled by aristocratic warrior elites, supported by farmers and specialised craftworkers, with rankings of kings, chiefs, knights, priests and clients. Roman writers describe a priestly class called druids, the subject of the reports of the Roman accounts of druids. Belief in a range of local gods, sacred places and offerings in water is attested by archaeology, and these are discussed under Celtic spirituality. Religion differed from region to region, and no single Celtic pantheon existed.
What science adds
Ancient DNA studies have shown movements of people across Europe during the Bronze and Iron Ages, but they describe ancestry, not language or culture. A person’s DNA does not reveal whether they spoke a Celtic language. Scholars generally treat the old picture of a single Celtic “invasion” or homeland as too simple, and many now see Celtic languages and styles spreading through networks of contact as well as migration.
Why it matters
Seeing the ancient Celts as many societies, sometimes allies and often rivals, explains why different Roman authors say different things and why modern claims about a unified Celtic people or a lost Celtic empire do not hold up. The Iron Age Celtic tribes are best understood through the detail of each region.