(Partially copy-pasting from an older but pretty much equivalent thread)
As in the title, what would happen to Gaul and Transalpine Roman Gaul if the Romans suffer a total defeat at the hands of the Gaulish coalition in Alesia?
Let's assume Labienus doesn't manage to asset the situation correctly or maybe having him dying earlier as a PoD during the campaign against Treviri; which would allow the rescue Gallic army to break the outer fortifications in conjuction of Vercevingetorix's army doing the same with inner walls, and Caesar may end his career as a trophy head, with his armoured body in display on the walls of an Arverni temple.
Gaul in the late -50's was a seemingly roughly pacified ensemble that revolted and ITTL would have crushed Romans, similarily to what happened at Teutobourg, with a probable similar trauma : that Romans would still keep control of conquered regions (besides Transalpina proper) as Aquitaine up to Garonne and Rhodanian corridor is plausible, but the revolt would probably generalized in most of Gaul, forcing a general withdrawal besides these points.
That said, culturally and politically, you'd have several changes ongoing.
The Gallic confederations and alliances may reach some importance comparable to contemporary Dacia, but romanisation was already a factor in Gaul since the beggining of the century : trade that exportated Roman way-of-life, civic structures as vergobrets possibly influenced by Roman politics, use of a common coinage based on roman denarii...etc. All of this participated to make Central Gaul part of the Roman sphere of influence, and even with a Roman defeat at Alesia, so would be Armorican and Northern Gaul.
Aedui would probably still keep their dominance in Central and Northern Gaul, while rivaled by Arverni. Geopolitically, the game's rules would have changed with, for exemple, the use of Roman tactics by Gauls, and the possible political changes.
Arverni would be the clear immediate winners, their old prestige renewed by the success of Vercingetorix's tactics. Without expecting a return of the old Arvernic Empire, we could see the re-establishment of an Arverni hegemony over south-western Gaul (with re-captation of former clients as Cadurci or Ruteni). Vercingetorix may turn as a "tyran "(in the classical sense, see below) but his "mandate" was about kicking Romans out and would not survive their defeat for what matters his power over several peoples : even during the siege IOTL, there were other Arverni leaders, some of which are more represented in found coinage than Vercingetorix.
In fact, I wouldn't be surprised to see re-emerging Gallic confederacies instituting a form of tyranny (in the ancient sense) as a leadership maintained trough popular support, especially in the power vaacum let by Caesarian shenanigants and conquests with new confederations araising in Gaul (especially in Belgium and Armorica) with formerly powerful peoples being incorporated into larger ensemble (Veneti power being significantly lowered, I could see them joining up with Armoricans, for exemple)
It's unlikely, giving the disunity of Gauls and the lack of real hegemonic power among the gallic coalition that you'd really see noticable campaigns (over than raids) in Transalpina : it's really not a given that most would even want to as many would want to prevent an Arverni takeover before anything else, would it be at the cost of a compromise with Rome : I'm especially thinking to traditional Roman allies as Aedui whom power and wealth depended from good relations with Rome.
Interestingly, the transrhenan exchanges would still be pretty much the thing they were : "Germans" (
we're actually talking of pretty much celtized peoples) would have an increased presence in Celtica, and the prosperous celtic ensembles in Southern Germany may not know the brutal decline they went trough historically.
Basically, Gallia ITTL may still means the land of the Celts (including Upper Danube, then) and not a region more or less forged by Caesar.
Of course, the already ongoing decline of druidism would certainly play a role on understanding changes of Gallic society : powerful peoples as Aedui let less places than what existed in the IInd century to Druids into everyday political life, as they remained an important political-social class, but less than they used to as a social group.
Now, socially, we see that we have Druids doing quite well in the Ist century. Diviciacos is an important person, representing Aedui in Rome, befriending Cicero's brother, and may have even been
vergobret of his people.
But as a druid, it's certainly quite opposite to the classical idea : no real restriction (dieterary or socially), openly speaking of druidic knowledge (which allows us to know about its depth, but also to see what was a restriction being largely breached).
A parallel with Roman religious roles (apart, maybe, flamines) that go broadly "civilized" and tied up with political institutions, may be interesting on this regard.
I don't think you could prevent a real Roman takeover of the region between Rhine and Pyrenees if they really want to do so, but Romans are going to have an harsher time : this time, Gauls would have learned the lesson of Caesar crushing one tribe after the other would have been taught, and more general opposition could follow.
Conquest of Gaul may be then more similar to what happened in Hispania : one step at the time. Granted, it was facilited by logistics, tough the really efficient road network and fertile farmlands, so it may not be as long as it happened there, but with such a conquest, *Gallo-Roman culture is going to be much different : more important celtic substrate, mostly, and the difference with the more romanized Transalpina is going to be even more obvious, maybe up to two distinct Gallo-Roman cultures.
Given Gaul was relatively more urban that Germany
The sociological, cultural and structural difference between the two sides of the Rhine is, while not entierly, largely a Caesarian creation : in fact, most of Rhineland and Danubian Germania was at least strongly celtized, and more of a mix of Celtic and Germanic features on which the Celtic elements was often dominant (Ariovist is a celtic name, for instance).
is it possible to have some sort of unified country ruling at least over half or a third Gallia Celtica? I imagine the Arverni have a shot at it given their participation in Alesia.
While you could see more unified confederacies,
as Aedui's IOTL (the existance of large archei in Gaul, with a complex network of obligations pre-existed Romans) I'd say greater unification,
regionally speaking, partly due to the aformentioned power vaacum.
Former allies of Caesar in Gaul, even if they joined the general rebellion (or didn't for that matter) would keep gains they obtained from this alliances : rivals being crushed, gain of territories or trade roads, etc.
Still, while the appearance of alliances/confederations may probably arise as I tried to point above, it's doubtful you'd see even within strong peoples as Aedui a tendency to centralisation with subordinated peoples disappearing as such at the benefit of the political center.
Would they have attempted that out of blue, Roman power and pro-Roman factions among Gauls, would certainly prevent that to happen.
Roman presence, influence and power (political, cultural and economical alike) is known in pre-conquest Gaul and may have helped Gaul adopting anti-monarchical, republican polities and as Caesar said (but also what archeological data seems to somehow support, while less bluntly than the Caesarian statement) : while Central-Eastern Gaul was well into Roman sphere of influence (I'm rambling, but that regional polities coinage was based on Roman coinage is more than telling), others regions seems to have escaped more and doesn't seems to have either vergobret institution, and to have kept more druidic features intact.
Making another parallel with Greeks, in the same way we had a distinction between
city-states and ethnic states in the archaic period, we may have a distinction between vergobret-doms and chiefdoms in pre-Roman Gaul (and that may be one of the reason to distinguish Belgae and others Celts on the left bank of the Rhine*)
Belgae/Brittons seems more conservative on this regard, they do seem to have known an assembly system as well (see how Ambiorix manages to lead a northern Gallic alliance). On the latter case, it's less sure that we couldn't, indeed, end with a more or less rough equivalent to High Kingship, in the Britton sense IMO, tough, as what happened with
Cogidubnos and possibily
Cunobelinos.
Meaning a form of cyclical chiefdom built over a former military alliance with institutions that could support their maintenance.
How long could it last without Roman support as with Cogidubnos, or existential threat as Cunobelinos, however... But, yes, it could create the way for an original mix of high-kingship/assembly system.
But Vercingetorix pulling a coup and going in a conquest spree?
Vercingetorix could use his prestige to topple part of Arverni elites, and enforce a tyrannic (again in ancient sense) rule but he would still have to deal with inner opposition : Epasnactos joined up with Rome (at the point pulling
Roman-inspired coinage) but was as well present in Alesia IOTL. Even with Vercingetorix's victory, he would be influential, and Vercingetorix trying to undergo a total takeover could lead to a civil war he wouldn't be sure to win.
He seems to have fought back for power, as a prelude to revolt, by being supported not by Arverni elites or core populations, but more excentric, peri-urban populations; rather than on the urban population of Nemossos strictly speaking.
Such inner contradictions would probably colour his rule. How exactly? I'm not sure, we don't know nearly enough about Gallic polities even in this region. Tough, it would probably impair its capacity to really gain power elsewhere, except maybe trough a "revanchist" program about reclientelising neighbouring peoples traditionally into Arverni sphere of influence.
But really, the core of the issue when it comes to central Gallic polities is that we're talking of sophisticated political structures : factions built on economical interests and familial lineages (as Vercingetorix had to fight his uncle, which is in a possibly matrilinear lineage point of view, full with information). Using the Renfew model, we have something in between of
complex chiefdoms and formative states in the IInd/Ist centuries BCE.
Maybe that the wars would help reinforcing a sense of regional unity and to give a more secular (more secular in these times, you know it better than I, still means pretty much ritualistic and religious, of course) pendant to druidic unifying features that were already declining.
We're talking long-range timeline tough, and Rome isn't going anywhere : their political/cultural/economical influence was really strong, and would likely continue to progress even without massive retiliation.