Germanic tribes, Gallic tribes, Illyrian tribes. What was a tribe? Was its structure formal or informal? How were the actions of the tribe, such as migrations and military actions, coordinated? What sort of importance did people place on their tribal affiliation? If the people in the next village over were a different tribe, how would that affect someone's view of them? Did these tribes have anything to do with the ruling structures that emerged, like kingships?
I know that's a lot to answer. I would be most appreciative even of a book recommendation.
That's rather a good question, especially as the term seems to appears regularily in question about Barbarian "tribes", in opposition to Greeks or Romans.
The first thing that must be stressed is that the name doesn't come from ancient sources : while Greeks or Romans can mention Barbarian peoples with different names, sometimes making a point to differenciate social frames (as with Caesar about Gauls), when they have to name peoples in a broad sense, they often resort to "natio" or "ethnè". Thing, is, these words can be used to name a small people or an entire "race" such as Germans or Gauls; both being defined by common ancestry, the size varying a lot; and without any real implication on the social sophistication : Aedui with their sophisticated state and Veneti living on the fringes of the known world could both be labelled as such.
Does that means that the word tribe didn't existed in Antiquity? It did and was used by Romans, to talk about their own social organisation, tribes being considered a traditional tripartite (tri-bus) subdivision of the original Roman people : Tites/Ramnes/Luceres extended during the royal period and early Republic, to 35 different tribes. While the original ones were often taken as coming from the three founding peoples (Latins, Sabins, Etruscans), a tribe was mostly defined trough civic lines, on which curia one belonged (tribes being divided in smaller ensembles) and participation (at least, possible participation) to assemblies.It's not clear at all if it was based on perceived genealogical ties, and it might rather be understood as a political identity.Athenian phylai (sometimes translated as tribes) seems to have been close enough from this, with the same blurry origin and without clear evidence it was associated to a genos, a group unified by ancestry.
We don't really know how ancient Barbarians named their own social organisation : thanks to Caesar, we know the main institutional frame was the "people" (civitas) made of pagi. Pagi were traditionally considered being the assembly of a tribe, but nowadays specialists rather argues that they include too many people and too great of a territory for that, Stefan Fichtl in particular proposing that Caesar's description of "fractions of pagi" might be a more accurate description of Gaulish tribes.Incidentally, thanks to Gallo-Etruscan and Gallo-Greek epigraphy and comparison with Old Irish, we might know the Gaulish word for tribe : *touta or *toouta, as in Toutatis "the tribal [god]". At least in some aspects, it seems that *tooutios/*toutios was based around a city, itself possibly defined by a common sanctuary : as such it is often translated as citizen.As in Rome, the tribal identity in Gaul seems to have played an important part into the making of a personal and local identity, but little in the political identity of Gauls as far as we can tell, the civitas (being understood as a political gathering of men in late Republican Latin, and not a city at least not in this context) became the main political institution, gathering pagi that themselves gathered fractions of pagi, these being possibly tribes. (Les Peuples Gaulois du IIIème au Iè siècle, Stefan Fichtl, 2005)
It's really unclear for early Germans. but what Caesar describe as the pagi of Ariovistos' confederation might be closer to be defined tribally : while Helvetii gather 5 pagi, Ariovist can count on more than one hundred. It's probably not due to an "ethnic" difference between Gauls and Germans (Germans of this period being often Celts, Celticized or partly made up of Celtic or Celticized peoples) than an institutional difference, Gaulish petty-states having possibly a separate and longer tradition of political build-up.
Still, an interesting set of inscrptions in Northern Britain might enlighten us a bit on the definition of a tribe for a Romanized German in the IInd century, especially what was found on the old Roman fort of Housesteads (RIB 1593 and RIB 1594)
Deo Marti Thincso et duabus Alaisiagis Bede et Fimmilene et N(umini) Aug(usti) Germ(ani) cives Tuihanti v(otum) s(olverunt) l(ibentes) m(erito)
To the god Mars Thincsus [of the Thing ]and the two Alaisiagae, Beda and Fimmilena, and to the Divinity of the Emperor the Germans, being tribesmen/citizens of the Tuihanti, willingly and deservedly fulfilled their vow.
Deo Marti et duabus Alaisiagis et N(umini) Aug(usti) Ger(mani) cives Tuihanti cunei Frisiorum Ver(covicianorum) Se(ve)r(iani) Alexandriani votum solverunt libent[es] m(erito)
To the god Mars and the two Alaisiagae and to the Divinity of the Emperor the Germans being tribesmen/citizens of the Tiuihanti the cuneus of Frisians of Vercovicium [Housesteads], styled Severus Alexander’s, willingly and deservedly fulfilled their vow.
Tihuanti being generally considered as a latinized Germanic name for the people around modern Twente, and the same known as Tubantes; we'd have a same consideration for citizenship/tribality, maybe around the idea of assembly (under the auspices of Mars of the assembly). The important part there would be the personal localized identity, and not necessarily the Frisian label that might be more Roman than Barbarian.
Of course, contact with Rome had certainly a great influence in Barbarian self-identity, not just materially but as well politically, and by the IIIrd century onward, Barbarians would refer to themselves trough a more broad identity, rather than tribal in this sense, as found in Pannonia.
Francus ego cives Romanus miles in armisI'm a Frankish citizen, Roman in military service.
For ancient sources, tribal simply doesn't have a relation to a degree of social sophistication, it is rather a matter of personal identity, shared with a small group and maybe less defined by blood than by assembly (or, rather, jointly with assembly), which would be made clear by the "adoption" of provincials citizens into Rome tribes.
It might have been, compared to broader assemblies and identities, an immediate and regular participation to the organisation of everyday life, maybe popular or partly democratic depending on the circumstances : how to manage sanctuaries, harvests, buildings, basically commune management decision, how to delegate but as well how to know and execute decisions taken at upper levels.
The usual meaning of tribe, in the modern sense, owes a lot to an extremely Eurocentric ethnology, issued from the Age of Discoveries and Colonial era; contacted peoples being considered as closer to barbarity than Europeans, as were Barbarian peoples to Romans. Using the word tribe, passed trough medieval Latin to call scattered Hebrew peoples in the Bible, it gave us the obnoxious "tribe".I won't dwell too much into a question that would probably, given its complexity and nature, have a better place and answer in r/AskAnthropology about a contemporary understanding of the concept, especially on an evolutionary model (although I find the rough state model proposed by Colin Renfrew interesting with an understanding of tribes as a relatively acephalic ensemble, Maurice Goudelier and Alain Testart being good reads on the general question too)
This question covers sooo much territory that I'm not even sure a good answer can get much more specific than this: modern Western society tends to think of 'tribe' as a very specific social grouping usually involving very primitive peoples (or peoples perceived to be so) who are all loosely related to one another and exist in a single village of humble dwellings. It's a realllly limited term that tends to get applied to a series of wildly-different types of societies and civilizations which have existed throughout history.
I'm not sure that there IS a single work or series of works that tackle the misapplication of the word 'tribe' and our mis-understanding of various kinds of social groupings that exist outside our limited understanding of our own cultural and social history, but there are a LOT of good books on Roman history that take on the idea of tribes in this way or that and sort of poke holes in our understanding of what makes a tribe a tribe and why we think what we do when we hear a group of people describe as a tribe. Peter Heather has several good books - Empires and Barbarians, The Fall of the Roman Empire - that do this, and Guy Halsall has some. The Ruin of the Roman Empire by James O'Donnell is a bit outside mainstream thinking about the fall of Rome, but a lot of his arguments are compelling, even if I think his conclusions are a bit of a stretch, and he has a good analysis of 'tribe.'
Basically, while we modern Westerners have a very limited and specific understanding of 'tribe,' tribe can mean a lot of different, sometimes over lapping things based on context. In some contexts, it can refer to a pre-urban, partially mobile socio-political structure in which groups of elders tend to dominate, sometimes with a chieftain or priest at the head, sometimes without, and in which the tribal members tend to share a limited sort of ethnic identity and kinship. These structures can be confined to a single village or settlement, or spread out across a larger territory and comprise multiple settlements. Sometimes the socio-political structure can contain lots of smaller, localized settlements with their own tribal chieftains or elders, who may not always agree with or be at peace with each other but can still recognize each other as belonging to the same tribe. There can even be 'kings' in some tribal contexts. This context can stretch so widely as to encompass not only the indigenous peoples of North America in the colonial period, but can also be applied to the 'tribes of Gaul', the early modern Highland Clans of Scotland, and the Germanic peoples who lived alongside the Roman Empire's borders in the first few centuries CE, so obviously the context and definition are somewhat imprecise. The major unifying thread, however, seems to be a very specific sort of pre-urban state, or deliberately non-urban state in which the distinguishing characteristics of urbanity are not present in the society.
In other contexts, 'tribe' can be a sort of socio-cultural designation that has almost nothing to do with political organization. Well into the late Roman Imperial period, despite being a very advanced, highly urbane society, the Romans still thought of themselves as members of different tribes. Tribal identification was still a part of a standard Roman funeral inscription. There had once been a political dimension to these tribal identities, but by the late Empire they were largely socio-cultural distinctions and not much more. There is evidence that many late Roman Britons still had tribal identities (Alcock's Brief History of Roman Britain), even if the tribes in question had dissolved as political units centuries before. In a way, the Scottish clans of today can be looked at in this context.
In the end, 'tribe' is a very imprecise and loaded term that we tend to uncritically apply to a lot of different sort of pre-urban or non-urban societies that we don't have more precise socio-political terminology for, as well as to a lot of highly urbane socio-cultural groupings that have no distinct political identities and only a loose sense of ethno-kinship.