Geoffrey mentions that Arthur conquered Dacia after pacifying Britain.
History of Dacia
There was a region in Europe called Dacia in ancient times. Dacia was a historical region located in the eastern part of modern-day Europe, corresponding roughly to present-day Romania, Moldavia, and parts of Bulgaria, Serbia, Hungary, and Ukraine.
The Dacians were an ancient Thracian people who lived in the territory known as Dacia. They had a rich and distinct culture, with their own language, religious beliefs, and social structures.
In 106 AD, the Roman Empire, under Emperor Trajan, launched a series of military campaigns against the Dacians. After two Dacian Wars, the Roman forces defeated the Dacians, and Dacia became a province of the Roman Empire.
Dacia became known as Roman Dacia (Dacia Romana) and was one of the northernmost provinces of the Roman Empire. The Romans introduced their administration, infrastructure, and culture to the region. Roman Dacia flourished under Roman rule, with the construction of roads, bridges, and cities. The Romans also introduced Christianity to the region.
In the third century AD, Roman Dacia came under pressure from various external threats, including invasions by Goths and Carpi. In the early fourth century, the Roman authorities decided to withdraw from Dacia due to the persistent attacks. The Roman administration and military presence gradually retreated south of the Danube River, leaving the region known as the Romanian Carpathians or Romania to the Romanized Dacian population.
Sources
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138
De Casibus Virorum Illustrium | Giovanni Boccaccio, 1355-1362