According to the book Gallic Wars by Caesar, when he went into Gaul there were some tribes and villages who almost immediately accepted Roman rule, while other areas rebelled. The tribes who rebelled were mainly decimated.
The question I have is: has anyone investigated the possibility that the tribes who readily accepted Roman rule may have been not ethnically part of the dominant culture of Gaul at the time?
From what I hear the main culture in Gaul was Celtic, although there was possibly German tribes too. But as far as the Celts are concerned they too were said to have invaded Gaul at some point in time subduing and perhaps to a degree displacing a previous population that would have been more indigenous to Gaul than the Celts.
And so what the Romans did in Gaul may have been a repetition of what the Celts did, militarily subduing a previous population.
So with this information it leads to the speculation that its possible that the tribes who readily accepted Roman rule may have viewed the Celts as invaders who had displaced other ethnic groups, and perhaps that could have been a reason why some tribes who accepted Roman rule were quick to accept Roman rule.
However this is speculation on my part, I don't know if anyone has investigated this sort of thing or if investigating it would even be possible. I have also heard that Celt referred more to a culture than an ethnic group, and the Celtic tribes were often hostile to each other.
As your question covers three aspects, namely ethnicity and culture in independent Gaul, their organisation and institutional strength, and the reason why Roman conquest and romanization were particularly quick, we'd have to covers each separately.
On the matter of ethnicity, first. What we'd call an ethnic group nowadays might not be the best tool in our mental toolbox to address ancient societies : we tend to consider ethnic groups as strongly particular groups distinguished by a core value, with groups being similar to their neighbours except for one of these core values (say religion), being treated as different ethnics. While the concept of ethnicity or relative ethnics might be more useful there, we can spare the expense and go right at the matter at hand.
It is important to stress right away that as far as we can tell, and in the conditions of its emergence during the Iron Age, Gaulish culture is largely indigenous, and is not a product of an elite invasion in recent history : while it have been argued since the XIXth century, archeologically, a continuity is rather observable since the Bronze Age in most of Gaul.Does that means that Gauls shared a similar culture, or represented a same ethnic group from Pyrenees to Rhine? It's quite more complex, and we shall look at the periphery first.
When Greeks landed in Gaul in the VIIth century BCE, they immediately contacted local peoples and called them Lyges or Ligurians, who spawned from the Arno to the Herault rivers.It would be tempting (and many were tempted since the XVIIIth century) to consider these Ligurians as a group of related people, culture-bearers of either a remaining larger Ligurian culture replaced by invading Gauls (Amédée Thierry's thesis), or an hold-out of a non-Celtic culture which had been tentatively identified with anything remotely plausible and then some (from Etrusceans to Phoenicians, including Basques, pre-IE, a first iteration of Celts, etc.).
The main problem is that while Ligurians does participate to a meta-civilisation of oppidae (which covered the Mediterranean arc from Spain to Italy), they are not really different from their neighbours in the hinterland, and tend to differ among themselves : it is why the term Ligurian is now more and more considered with extreme caution and rarely an option of identification.But if Ligurians, in terms of material culture, can't be differentiated from Celtic or Italic peoples, what would have they been?
A first element of answer is in their name : it is now assumed that Lyges is a Greek word for "Howlers", comparable to Barbarian in the sense it names people unable to speak properly, because Greeks were just this polite and tactful. The Ligurian shore, the Ligustikè, would then be a Barbarian shore defined first and first-most from a Greek geo-social point of view. Hecate of Milet, by describing Ligurians as one of the "outermost" people with Scythians and Ethiopians (themselves not that well defined either) is making a similar conceptual point (and it's interesting to note that Ligurians hold the same place that Celts would have for Greeks in this matter).Greek scholars doesn't give us much about what these Ligurians were, except telling that Greeks settled among them when founding Massalia (the Ligurian tribe they dealt with bearing the quite obviously Gaulish name of Segobriges).By the Vth century BCE, Greeks begin to use a different name for these peoples, at least in Gaul : Celto-Lyges, or Celto-Ligurians. It was traditionally understood as the result of a Celtic invasion or takeover by a Gaulish elite : then again, not only it is not attested in tradition, there's no real archeological evidence for this.Dominique Garcia, Sophie Colin Bouffier and Philippe Bossinot (following the idea proposed by Roger Dion) if Ligurians are not a precise people but the barbarous societies Greek encountered there, the addition of Celto- might be a tentative to refine the definition with the new knowledge obtained by Massaliots.
At first glance, it only pushes the problem into the definition of Celts, that u/Typology well put there : without entering the debate of the accuracy of the name Celt in modern historiography, let's focus on what the word meant in the Vth century for Greeks, and maybe for Gauls. *Keltoi might have been a Ligurian people, or a Ligurian confederation of people as Salyes, Cavares, etc. that would have formed out of the necessity to deal with Massalia in matters of trade, exchanges but also war (as local peoples began to push back Massalia out of its coastal hinterland at the same period). As such an alliance (*Keltoi would have meant something like "Associates" in this context) had obvious benefits, it would have met with a regional success, with most people there joining the Celtic bandwagon.From there, Greeks would have acknowledged the existence of a specific group, maybe different culturally from Ligurians in Corsica or Italy, and would have called their neighbours "Celto-Ligurians" to refine their point of view just like we use "Amerindian" instead of "Indian".From this point, Celt had been used by Greeks to define the whole of the barbarous hinterland, just like Ligurian had been used for the whole barbarous shore. In the same time, the use of Celt to name not only the general alliance, but the grouping of related people might have led to its systematisation in a good part of Gaul although never, regardless what Caesar said, used as a cultural endonym.
Does that mean there was not any Ligurian specificity? Probably not, but it's hard to assert which ones due to both the lack of evidence from one hand, and nonsensical affirmations made since the XVIIIth century. Linguistically, the -i/oscos and -ica radicals might be indication of some particularity (although they are found elsewhere in Gaul) and it's not impossible that a stronger non-Celtic substrate might have influenced the local language. But, from what we know of both Gaulish language, and the study of Celto-Ligurians names and place names, it seems unmistakably Gaulish.
Cultural differentiation among Celto-Ligurians took rather the form of a much stronger Hellenistic influence in urbanism, the use of a Greek script to write Gaulish, and a lesser grasp of Druidism on the artistic representation (while Gaulish art is strongly aniconic generally). Celto-Ligurians might have realized their difference with other Gaulish peoples, and have asserted their specific interests in their relation with Massalia (either peaceful or hostile), and have formed their own assemblies to deal with local matter among them. It remains highly speculative, but not unthinkable in the light of Gaulish institutions we'll talk about later.While considered as Celtized, some Ligurian peoples or tribes in Italy are generally not lumped together with Gaulish Celto-Ligurians in modern historiography.
