Short answers, pulled from the story.
Gallia Belgica emerged in 22 BC as the official province following a census conducted in 27 BC. Emperor Augustus ordered Marcus Agrippa to divide Gaul into three new regions, and this administrative restructuring created the province.
Julius Caesar led a military campaign in 57 BC that defined the northern boundary of Gaul. He described Belgic Gaul as one of three parts separated from Celtic Gauls by the Marne and Seine rivers.
Cities like Cassel, Bavay, Thérouanne, and Arras became district capitals known as civitates. The capital initially sat at Reims according to geographer Strabo before later shifting to Trier.
A disastrous Rhine crossing occurred in 406 under pressure from Huns. Vandals, Alans, and Suebi entered Gallia Belgica via the Moselle valley and destroyed large parts of the northern region before moving toward Hispania.
Emperor Diocletian split Belgica into two distinct entities: Belgica Prima and Belgica Secunda. The Meuse river marked the approximate border between these new divisions with Belgica Prima centered on Augusta Treverorum.