Yeah I definitely fall into the 'military historian' category (bonus points for calling that I've read Goldsworthy) so it's really interesting to get the perspective of a classicist such as yourself. tbh most of my knowledge about the Roman military is from the Principate and Dominate around the 1st-5th centuries AD and I've never understood the mechanisms for raising armies in the Late Republic. Armies were raised for a campaign and then dismissed but I know there were also some means of keeping veterans around (didn't Pompey keep some of his veterans on garrison duty during his time in politics?). Anyway I found your analysis of the reforms of Marian vs Sulla vs Augustus to be really informative.

I might have screwed up in expressing my understanding of Gallic (I know that's kind of an umbrella term but for the purposes of this discussion, in the military context, we'll take it to mean the Helvetii or other sub-groups who fought in a phalanx and so required as much discipline, organisation and defensive equipment as possible) and other 'barbarian' armies. Obviously social status doesn't determine military capability. You already gave some good examples of that, although iirc recent scholarship has moved away from the idea of 'peasant-archers' at Agincourt as the English Yeoman were actually a kind of rural middle class (it's hard to transpose modern class systems onto the middle ages obvs). Anyway my point was that in most warrior societies I've studied (I'm including the 'barbarians' faced by Rome although I honestly don't know much about them) military forces were based around powerful magnates and their bands of professional, well (if not uniformly) equipped and trained warriors. The numbers in an army would then be made up by lower class non-professionals (although contrary to popular belief these weren't peasant cannon-fodder forced to fight at sword-point, but more of a middle class who had rudimentary training and equipment). The problems with these armies was the lack of centralized command as the core of the army, the warrior households, were loyal to their chieftain/lord etc. at the expense of the ultimate commander. I found John France's analysis of this in Western Warfare in the Age of the Crusades to be really interesting and well fleshed-out, although it's obviously outside the time period under discussion I think the principal still stands. There was also a major disparity in terms of equipment and training between the professional core and the rest of the army. My understanding of the 'Gallic' armies was that while the warrior elite had all the trapping you've described; mail (which by several accounts was actually invented by the Gauls and adopted by the Romans), swords and tower shields the rest of the army normally made do without any body armour, fighting only with a small shield and simple spear. I find it hard to believe that a rural, fragmented society such as Gaul, without a standing army could equip an entire army (especially those as massive as those that opposed Caesar) in Mail. Although now that I'm talking about it with you I'm not sure how the Roman did it before the Empires standing army either.

Also about the troop numbers: I understand how urbanized societies like Roman and the Greek city-states could effectively call up large armies but I'm not sure if the much less centralized, less urbanized Gauls could do the same.