She reads Caesar as implying a "transition zone" of mixed ethnicity and ancestry for the Menapii, Nervii, and Morini, all living in the northwest of the Belgic region, neighbours to the Germani cisrhenani in the northeast.) It seems that, whatever their Germanic ancestry, at least some of the Belgic tribes spoke a variety of the Celtic Gaulish language as their main language by Caesar's time, and all of them used such languages in at least some contexts.Luc van Durme summarizes competing evidence of Celtic and Germanic influence at the time of Caesar by saying that "one has to accept the rather remarkable conclusion that Caesar must have witnessed a situation opposing Celtic and Germanic in Belgium, in a territory slightly more to the south than the early medieval Romance-Germanic language border", but van Durme accepts that Germanic did not block "Celticisation coming from the south" so "both phenomena were simultaneous and interfering".Some peoples in Britain were also called Belgae and O'Rahilly equated them with the Fir Bolg in Ireland.The Belgae gave their name to the Roman province of Gallia Belgica and, much later, to the modern country of Belgium; today "Belgae" is also Latin for "Belgians".The sound changes described by "Grimm's law" appear to have affected names with older forms, apparently already in the second century BC.Strong evidence for old Celtic placenames, though, is found in the Ardennes and to the south of them.
Caesar's book The Gallic Wars begins: "All Gaul is divided into three parts, one of which the Belgae inhabit, the Aquitani another, those who in their own language are called Celts, in ours Gauls, the third.
The Belgae attacked over the river, but were repulsed after a fierce battle.
Realising they could not dislodge the Romans and aware of the approach of the Aedui into the lands of the Bellovaci, the Belgae decided to disband their combined force and return to their own lands.
The medieval Gesta Treverorum compiled by monks of Trier claims that the Belgae were descendants of Trebeta, an otherwise unattested legendary founder of Trier, the Roman Augusta Treverorum, "Augusta of the Treveri".
Later, Tacitus mentioned a tribe called the Tungri living where the Germani cisrhenani had lived, and he also stated that they had once been called the Germani, (although Caesar had claimed to have wiped out the name of the main tribe, the Eburones). He writes that the Belgae were conspiring and arming themselves in response to his earlier conquests; to counter this threat, he raised two new legions and ordered his Gallic allies, the Aedui, to invade the territory of the Bellovaci.