I am studying the La Tène era Celts and it seems that although they had all of the cultural similarities and trade relations of a unified people, they remained a patch work of waring tribes eventually overtaken by the more efficiently unified Rome. It seems to me that the formation of a state requires more than cultural and economic influences, but I cant figure out what.
There are a number of historians/anthropologists would contest that there was not a unified culture. Hopefully someone can actually talk about this at length, but I would like to suggest that you take a look at this book, which is an excellent introduction to these problems: http://www.worldcat.org/title/beyond-celts-germans-and-scythians-archaeology-and-identity-in-iron-age-europe/oclc/46433475