Question

how did the small tribes of earlier period give way to large kingdom

Answer 1

Urbanization is what moves polities out of tribalism and into centralized, more permanent civil structures. It also gives a strong push out of “kingship” and into republicanism. But that’s not half the story.

Tribalism

If your question begins with “small tribes”, then much of what you think you know about tribes is probably wrong. Tribal states have historically been some of the largest, most expansive and competitive in their day. Before professional armies in the 18–19th centuries, the chief problem of any ruler was finding people who could fight, and fight well: peasants don’t learn to fight, can’t really be trained (or they stop being peasants, and historically it was near impossible to economically support mass training even if you knew how to do it). So you have to find people who know how to use arms as part of their way of life. This is why, for example, England had a formidable archery corps in the middle ages, unlike the rest of Europe: because the yeomen of the Welsh marches had indigenous archery skills the king could put to good use against the French.

Enter tribes. Not all tribes are created equal, but most are provided with traits that made them enviable fighting-forces: (a) an internal, natural solidarity as a group, (b) skill with their traditional weapons, which were for many their source of sustenance, (c) a more vigorous lifestyle/physique than their settled counterparts, (d) poverty: the motivation to go to war to take other people’s money. Combine these, and it makes sense why tribal militaries dominated half the world at any point in time, usually becoming ruling classes over the more “civilized” indigenous people.

Nor were tribes limited to “small” kingdoms, per se. In fact the opposite is true: the old tribal kingdoms were, in their total expanse, much larger than their settled successors. Under the barbarian tribes’ kingships, before feudalism really started early medieval Europe had more wide-reaching kingdoms than it would develop until modern times…



…Achaemenian Persia was immense, compared to its successors…



…and in the Steppe, nomadic tribes ruled realms the size of continents.



Civilization

What maps like these don’t show is that such vast realms were far more disunited than they look. As a consequence, they were impermanent and prone to fragmentation and/or civil war between their constituent tribes. While they often included urbanized areas - both the Franks and the Persians ruled thickly urbanized nodes (Italy and Mesopotamia, respectively) of the civilized peoples that preceded their empires, and whose languages they used (Latin and Aramaic), and this contributed to stabilize their empire - make the ruler less “first among equal” chieftains and more a true sovereign surrounded by the pomp and circumstance of an older, respected world (Rome, Babylon…), everything outside those locales grew increasingly less solid, still divided into autonomous dynastic duchies/satrapies with their own laws, tribal nations, and desire for autonomy.

So, empires were really works in progress: over centuries, urban foundations would spring up over the hinterland of these old states. In the process, the large old states would often break up into less coherent local authorities, as cities at once make the provinces less isolated and more connected, but also give each province more individual “pull” to come apart. At the same time, growing urbanization gives kings more and more ability to use the full resources of the developed lands. From a thin ruling class of horsemen forcing tribute and military aid out of a large number of vassal communities and smaller tribes, the next generation of polities develop bureaucracies, develop more intensive taxation, and become more active, permanent institutions with a more diverse military and social support base.

France is a good example of this, vis-a-vis the German-Roman empire. The latter was the major European power until about the 13–14th century: and consisted of an uneven mix of the highly urbanized Italian lands (which had a tendency to become a gaggle of independent, anti-German republics), and the tribal German and Slavic countries/duchies, each with its own agenda. The Emperor was the most powerful ruler in Europe, but had to juggle a monstrous labyrinth of competing interests: to keep Italy (the real prize, but foreign to him and with more ability to resist) he risked losing Germany, but to keep Germany meant to give up what made the Empire powerful and exceptional power of its day.






An interesting counterpart to Europe’s hard, difficult to maintain military shell “cracked”, the devastation was tremendous. Chinese historical demography is divided between high peaks and low valleys, with population peaking under strong dynasties and threatened by extinction whenever the dynastic machine failed (often due to the overpopulation it itself produced).