Gallia Belgica
The area was conquered in 57 BC by Julius Caesar.  Emperor Diocletian changed the Gaulish provinces around 300 AD. He split Belgica into two provinces: Belgica Prima and Belgica Secunda. Belgica Prima had Treveri (Trier) as its main city and was the eastern part. The border between Belgica Prima and Belgica Secunda was along the River Meuse.