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In ancient geography, especially in Roman sources, Dacia was the land inhabited by the Dacians or Getae as they were known by the Greeks—a branch of the Thracians north of the Haemus range.
Dacia was bounded in the south approximately by the Danubius river (Danube), in Greek sources the Istros, or at its greatest extent, by the Haemus Mons (the Balkan Mountains). Moesia (Dobrogea), a region south of the Danube, was a core area where the Getae lived and interacted with the Ancient Greeks. In the east it was bounded by the Pontus Euxinus (Black Sea) and the river Danastris (Dniester), in Greek sources the Tyras. But several Dacian settlements are recorded between the rivers Dniester and Hypanis (Bug River), and the Tisia (Tisza) to the west.
At times Dacia included areas between the Tisza and the Middle Danube. The Carpathian Mountains were located in the middle of Dacia. It thus corresponds to the present day countries of Romania and Moldova, as well as smaller parts of Bulgaria, Serbia, Hungary, and Ukraine.
Dacians (or Getae) were North Thracian tribes. Dacian tribes had both peaceful and military encounters with other neighboring tribes, such as Celts, Ancient Germanics, Sarmatians, and Scythians, but were most influenced by the Ancient Greeks and Romans. The latter eventually conquered, and linguistically and culturally assimilated the Dacians.
A Dacian Kingdom of variable size existed between 82 BC until the Roman conquest in 106 AD. The capital of Dacia, Sarmizegetusa, located in modern Romania, was destroyed by the Romans, but its name was added to that of the new city (Ulpia Traiana Sarmizegetusa) built by the latter to serve as the capital of the Roman province of Dacia.
Source : http://en.wikipedia.org/wiki/Dacia