Dacia was situated in the loop of the lower Danube (see danuvius), consisting mainly of the plateau of Transylvania, but extending in a wider sense eastwards to the Sereth and north to the Vistula. The Dacians were an agricultural people, but under the influence of Celtic invaders (see celts) in the 4th cent. bce they absorbed Celtic culture and developed the gold, silver, and iron mines of the Carpathians. From c.300 bce they traded with the Greeks, who frequently confused them with the Getae, by way of the Danube; from the 2nd cent. they also had relations with the Greek cities of Illyria (see illyrii) and Epirus via Roman traders, seeking slaves. Their chief import was wine.
The separate Dacian tribes were united by Burebistasc.60 bce and under him they conquered Celtic and Illyrian peoples to the south and west, threatening Roman Macedonia. After his death the power of Dacia declined because of internal struggles. For a time the Dacians were regarded as a serious threat. Caesar was planning a campaign against them before his death (Suet.Iul. 44. 3). Some years later Octavian (App.Ill.23), also fearing a possible alliance between Antony (M. Antonius (2)) and the Dacians, sought a marriage alliance with Cotiso, one of the rival Dacian kings (Suet.Aug.63). Under Augustus the Dacians caused few problems, but their military power revived under Decebalus with victories over Oppius Sabinus ( ce 85) and Cornelius Fuscus (86). After a Roman victory at Tapae (SW Transylvania) Domitian made peace, recognizing Decebalus as a client ruler (see client kings). Conquest of Dacia was effected by Trajan in the First and Second Dacian Wars (101–2, 105–6, cf. Cass. Dio 58. 6–14). Decebalus' stronghold Sarmizegethusa (mod. Gradiştea Muncel south of Oraştie, one of several Dacian citadels which have been explored) was taken and destroyed. These campaigns are depicted on the spiral frieze of Trajan's Column.
Under Hadrian (117–18) Dacia lost one of its legions while Roman auxiliary garrisons were withdrawn from the plains of the Banat to the west and Wallachia to the east. The remaining territory was divided into three provinces, Upper Dacia comprising the heartlands of Transylvania under a praetorian legate and two lesser commands under procurators, Lower Dacia in the south-east, and Porolissensis in the north-west. Later in the century (c.168) the provinces were joined again as Tres Daciae under a consular legate.
There was a great influx of people from other provinces into Dacia, especially from Illyricum. Some came as skilled miners, e.g. Dalmatians in the gold mines at Alburnus Maior (Roşia Montana). More than a dozen cities were established; the most important were the coloniae at Ulpia Traiana (Sarmizegethusa) and Apulum. As a result of the Gothic invasions (mid-3rd cent.) Dacia was evacuated under Aurelian (270), and the name transferred to a new province along the south bank of the Danube.