The history of Rome and Dacia is another example of friction at the edge of the Empire causing a confrontation with people who refused to be subjugated. It took the Romans nearly twenty years to defeat Dacia once hostilities broke into the open.
With the eastern European frontier the Romans employed, as elsewhere, the same strategy. First, they required that the frontier facilitate traffic flowing between the various parts of the Empire. Second they rejected areas that were difficult to settle. Third they specified that the frontier include lands that could provide food and natural resources for the Romans settled there.
The tribe of Dacians was located north of the Danube River in southeastern Europe in what is today Romania and Moldova. At various times in their history, The Dacians, called Getae by the Greeks, expanded south of the Danube to the edge of the Balkan Mountains in what is now modern Bulgaria. The Dacians had a propensity for centralization which was rare for the peoples of the region and this trait made them a dangerous adversary for any power operating in the vicinity of the Danube.
During the first century AD, before Trajan, the Roman frontier in southeast Europe had its northern boundary at the Danube River stretching from Vienna all the way to the Black Sea. The Danube was fortified along its entire length with large forts, watchtowers, and auxiliary units assigned to reconnaissance, while two naval fleets, the Classis Pannonica and Classis Moesia, patrolled the river itself.
The map above shows the geography of the Balkans area.
Dacian raids against Rome were somewhat controlled under Augustus through reprisal operations. Tiberius tried diplomacy but was unsuccessful, possibly because the Dacians possessed gold and refused to be bought off. Then, during the middle of the first century AD, the Romans used Sarmatian Lazyges as a buffer by having them occupy areas Between the Tisza River and the Danube. The Lazyges, a nomadic people, were willing to take as payback for their territorial commitment Roman help in suppressing internal rebel activity.
In 85/86, during the reign of Domitian, the Dacians came together under the rule of Decebalus and became more belligerent. A Dacian attack on Moesia in 87 led to a Roman pursuit across the Danube and a serious Roman defeat, later avenged by Roman victory at Tapae in 88. Domitian had designs on attacking the Dacian capital at Sarmizegethusa but delays caused by matters needing Roman attention elsewhere resulted in a lost opportunity. By the time Rome turned its attention back to Dacia, the client kingdoms of the Danube had crumbled, making a large scale attack no longer possible. Uncertainty in the region required that Domitian treat with the Dacians, including the offering of a technical aid program, so things remained quiet along the eastern Danube up until the time of Domitian’s assassination in 96 AD.
Trajan attacked Dacia in the 101-2 period, defeated Decebalus, and exacted severe concessions on the losers. Almost immediately, the terms of the treaty were abused and a second war commenced in 105. This time Trajan laid siege to Sarmizegethusa and destroyed it. Decebalus committed suicide to avoid Roman capture.
Trajan made Dacia a Roman province extending its land as a deep wedge north from the Danube, a design intended to separate the local tribes and decrease their ability to organize together. This Dacian wedge survived until 270, when Aurelian abandoned the territory to conserve military resources.