Gallia Belgica ("Belgic Gaul") was a province of the Roman Empire located in the north-eastern part of Roman Gaul, in what is today primarily northern France, Belgium, and Luxembourg, along with parts of the Netherlands and Germany.
In 50 BC, after the conquest by Julius Caesar during his Gallic Wars, it became one of the three parts of Gaul (Tres Galliæ), the other two being Gallia Aquitania and Gallia Lugdunensis. An official Roman province was later created by emperor Augustus in 22 BC. The province was named for the Belgae, as the largest tribal confederation in the area, but also included the territories of the Treveri, Mediomatrici, Leuci, Sequani, Helvetii and others. The southern border of Belgica, formed by the Marne and Seine rivers, was reported by Caesar as the original cultural boundary between the Belgae and the Celtic Gauls, whom he distinguished from one another.
The province was re-organised several times, first increased and later decreased in size. Diocletian brought the northeastern Civitas Tungrorum into Germania Inferior, joining the Rhineland colonies, and the remaining part of Gallia Belgica was divided into Belgica Prima in the eastern area of the Treveri, Mediomatrici and Leuci, around Luxembourg and the Ardennes, and Belgica Secunda between the English channel and the upper Meuse.
The capital of Belgica Prima, Trier, became an important late western Roman capital.
Roman conquest
In 57 BC, Julius Caesar led the conquest of northern Gaul, and already specified that the part to the north of the Seine and Marne rivers was inhabited by a people or alliance known as the Belgae. This definition became the basis of the later Roman province of Belgica. Caesar said that the Belgae were separated from the Celtic Gauls to their south by "language, custom and laws" (lingua, institutis, legibus) but he did not go into detail, except to mention that he learnt from his contacts that the Belgae had some ancestry from east of the Rhine, which he referred to as Germania. Indeed, the Belgian tribes closest to the Rhine he distinguished as the Germani cisrhenani. (Strabo stated that the differences between the Celts and Belgae, in language, politics and way of life was a small one.) Modern historians interpret Caesar and the archaeological evidence as indicating that the core of the Belgian alliance was in the present-day northernmost corner of France; the Suessiones, Viromandui and Ambiani as well perhaps as some of their neighbours who lived in the area, Caesar identified as Belgium or Belgica. These were the leaders of the initial military alliance he confronted, and they were also more economically advanced (and therefore less "Germanic" according to Caesar's way of seeing things) than many of their more northerly allies such as the Nervii and Germani Cisrhenani.
Apart from the southern Remi, all the Belgic tribes allied against the Romans, angry at the Roman decision to garrison legions in their territory during the winter. At the beginning of the conflict, Caesar reported the allies' combined strength at 288,000, led by the Suessione king, Galba. Due to the Belgic coalition's size and reputation for uncommon bravery, Caesar avoided meeting the combined forces of the tribes in battle. Instead, he used cavalry to skirmish with smaller contingents of tribesmen. Only when Caesar managed to isolate one of the tribes did he risk conventional battle. The tribes fell in a piecemeal fashion and Caesar claimed to offer lenient terms to the defeated, including Roman protection from the threat of surrounding tribes. Most tribes agreed to the conditions. A series of uprisings followed the 57 BC conquest. The largest revolt was led by the Bellovaci in 52 BC, after the defeat of Vercingetorix. During this rebellion, it was the Belgae who avoided direct conflict. They harassed the Roman legions, led personally by Caesar, with cavalry detachments and archers. The rebellion was put down after a Bellovaci ambush of the Romans failed. The revolting party was slaughtered.
Formation under Augustus
Following a census of the region in 27 BC, Augustus ordered a restructuring of the provinces in Gaul. Therefore, in 22 BC, Marcus Agrippa split Gaul (or Gallia Comata) into three regions (Gallia Aquitania, Gallia Lugdunensis and Gallia Belgica). Agrippa made the divisions on what he perceived to be distinctions in language, race and community – Gallia Belgica was meant to be a mix of Celtic and Germanic peoples. The capital of this territory was Reims, according to the geographer Strabo, though later the capital moved to modern-day Trier. The date of this move is uncertain.