Trajan's Dacian Wars in the context of "Trajan's Column"

Romania is a country located at the crossroads of Central, Eastern and Southeast Europe. It borders Ukraine to the north and east, Hungary to the west, Serbia to the southwest, Bulgaria to the south, Moldova to the east, and the Black Sea to the southeast. It has a mainly continental climate, and an area of 238,397 km (92,046 sq mi) with a population of 19 million people. Romania is the twelfth-largest country in Europe and the sixth-most populous member state of the European Union. Europe's second-longest river, the Danube, empties into the Danube Delta in the southeast of the country. The Carpathian Mountains cross Romania from the north to the southwest and include Moldoveanu Peak, at an altitude of 2,544 m (8,346 ft). Bucharest is the country's capital, largest urban area, and financial centre. Other major urban areas include Cluj-Napoca, Timișoara, Iași, Constanța and Brașov.

Settlement in the territory of modern Romania began in the Lower Paleolithic, later becoming the Dacian Kingdom before Roman conquest and Romanisation. The modern Romanian state was formed in 1859 with the unification of Moldavia and Wallachia under Alexandru Ioan Cuza, becoming the Kingdom of Romania in 1881 under Carol I of Romania. Romania gained independence from the Ottoman Empire in 1877, formalised by the Treaty of Berlin. After World War I, Transylvania, Banat, Bukovina, and Bessarabia joined the Old Kingdom, forming Greater Romania, which reached its largest territorial extent. In 1940, under Axis pressure, Romania lost territories to Hungary, Bulgaria, and the Soviet Union. Following the 1944 Romanian coup d'état, Romania switched sides to join the Allies. After World War II, it regained Northern Transylvania through the Paris Peace Treaties. Under Soviet occupation, King Michael I was forced to abdicate, and Romania became a socialist republic and Warsaw Pact member. After the uniquely violent Romanian revolution in December 1989, Romania began a transition to liberal democracy and a market economy.

Dacia (/ˈdeɪʃə/, DAY-shə; Latin: [ˈd̪aː.ki.a]; Ancient Greek: Δακία) was the land inhabited by the Dacians, its core in Transylvania, stretching to the Danube in the south, the Black Sea in the east, and the Tisza in the west. The Carpathian Mountains were located in the middle of Dacia. It thus roughly corresponds to present-day Romania, as well as parts of Moldova, Bulgaria, Serbia, Hungary, Slovakia, Czech Republic, Poland and Ukraine.

A Dacian kingdom that united the Dacians and the Getae was formed under the rule of Burebista in 82 BC and lasted until the Roman conquest in AD 106. As a result of the wars with the Roman Empire, after the conquest of Dacia, the population was dispersed, and the capital city, Sarmizegetusa Regia, was destroyed by the Romans. However, the Romans built a settlement bearing the same name, Ulpia Traiana Sarmizegetuza, 40 km away, to serve as the capital of the newly established Roman province of Dacia. A group of "Free Dacians" may have remained outside the Roman Empire in the territory of modern-day Northern Romania until the start of the Migration Period.

Roman Dacia (/ˈdeɪʃə/ DAY-shə; also known as Dacia Traiana (Latin for 'Trajan’s Dacia'); or Dacia Felix, lit. 'Fertile Dacia') was a province of the Roman Empire from 106 to 271–275 AD. Its territory consisted of what are now the regions of Oltenia, Transylvania and Banat (today all in Romania, except the last region which is split among Romania, Hungary, and Serbia). During Roman rule, it was organized as an imperial province on the borders of the empire. It is estimated that the population of Roman Dacia ranged from 650,000 to 1,200,000. It was conquered by Trajan (98–117) after two campaigns that devastated the Dacian Kingdom of Decebalus. However, the Romans did not occupy its entirety; Crișana, Maramureș, and most of Moldavia remained under the Free Dacians.

After its integration into the empire, Roman Dacia saw frequent administrative reorganization. In 119 under Hadrian, it was divided into two departments: Dacia Superior ("Upper Dacia") and Dacia Inferior ("Lower Dacia"; later named Dacia Malvensis). Between 124 and around 158, Dacia Superior was divided into two provinces, Dacia Apulensis and Dacia Porolissensis. The three provinces would later be unified in 166 and be known as Tres Daciae ("Three Dacias") due to the ongoing Marcomannic Wars. New mines were opened and ore extraction intensified, while agriculture, stock breeding, and commerce flourished in the province. Roman Dacia was of great importance to the military stationed throughout the Balkans and became an urban province, with about ten cities known and all of them originating from old military camps. Eight of these held the highest rank of colonia. Ulpia Traiana Sarmizegetusa was the financial, religious, and legislative center and where the imperial procurator (finance officer) had his seat, while Apulum was Roman Dacia's military center.

Mauri (from which derives the English term "Moors") was the Latin designation for the Berber population of Mauretania, located in the west side of North Africa on the shores of the Mediterranean Sea, Mauretania Tingitana and Mauretania Caesariensis, in the North of present-day Morocco and north western Algeria.

Sarmizegetusa Regia (also known as Sarmisegetusa, Sarmisegethusa, Sarmisegethuza; Ancient Greek: Ζαρμιζεγεθούσα, romanized: Zarmizegethoúsa) was the capital and the most important military, religious and political centre of the Dacians before the wars with the Roman Empire. Built on top of a 1200 m high mountain, the fortress, consisting of six citadels, was the core of a strategic and defensive system in the Orăștie Mountains (in present-day Romania).

Sarmizegetusa Regia should not be confused with Ulpia Traiana Sarmizegetusa, the Roman capital of Dacia built by Roman Emperor Trajan some 40 km away, which was not the Dacian capital. Sarmizegetusa Ulpia was discovered earlier, was known already in the early 1900s, and was initially mistaken for the Dacian capital, a confusion which led to incorrect conclusions being made regarding the military history and organization of the Dacians.

The Free Dacians (Romanian: Dacii liberi) is the name given by some modern historians to those Dacians who remained outside, or emigrated from, the Roman Empire after the emperor Trajan's Dacian Wars (AD 101-6). Dio Cassius named them Dakoi prosoroi (Latin: Daci limitanei) meaning "neighbouring Dacians".

A population of Dacians existed on the fringes of the Balkan Roman provinces, especially in the eastern Carpathian Mountains, at least until about AD 340. They were responsible for a series of incursions into Roman Dacia in the period AD 120-272, and into the Roman Empire south of the Danube after the province of Dacia was abandoned by the Romans around AD 275.