The years 1400 - 1600 saw, at least in Europe, the emergence of powerful monarchs and princes in what later came to be nation-states.
De Jouvenel says that 'sovereignty', in the sense of the over-arching power of the sovereign, was a 17th century construction, and that all previous societies regarded themselves as being assemblages of individuals subject to a common law, which applied to the sovereign (if there was one) as much as to any other citizen. He adds that the State became personified only in the 19th century: where we now say 'France', and give it personality, Romans used to say, according to the date of the speaker, either 'the people and commons of Rome', or 'the Senate and people of Rome'.
Tracing the growth of the power of the monarch and then the State, de Jouvenel points out that in mediaeval times such power was severely tramelled by the 'Lex Terrae', the customs of the country, 'which was thought of as a thing immutable'. 'And when the English Barons uttered their Nolumus leges Angliae mutari (We object to changes in the laws of England), they were only giving vent to the general feeling of the time.'
Prior to the 16th century, the consciousness of all individuals other than very well educated ones was unaffected by direct delivery of printed ideas, although the parson in his pulpit and the school-teacher of course could and did deliver ideas and morals in the vernacular. Benedict Anderson13 points to the invention of printing, and the consequent spread of demotic national 'print languages' which replaced Latin as being the source of nationalism and the concomitant emergence of national group feelings in the individual psyche. The language of print in Europe, until the arrival of Martin Luther in the 16th century, had been exclusively Latin. After that, printing in the vernacular spread rapidly as a way of educating, informing, controlling masses of people who would have been beyond the reach of copyists. The combination of Protestantism and print-capitalism led to the establishment in the 17th century of Europe's first significant non-dynastic 'nation' states in the Dutch Republic and the English Commonwealth.
Whatever the exact mechanism, the expansion of individual consciousness to embrace 'national' feelings is evident, and it had many unpleasant consequences, alongside a few good ones. It's not unreasonable to date the decay of group-driven society from the fact of the emergence of the nation state. And without the development of patriotism that resulted from the emergence of national consciousness, the financing and bloodshed of the national wars of the 18th to 20th centuries would hardly have been possible.
This is not to say that nations as such had not existed prior to the 15th century. Kropotkin points to Merovingian France and 12th century Russia as being national in character, but: 'These nations . . . were nevertheless kept together by nothing else but a community of language, and a tacit agreement of the small republics to take their dukes from none but one special family.'
Anderson points out that nation states in South America (and later in Africa) largely followed the contours of the colonial administrative districts which had preceded them. It's easy to see that 'national print languages' and accompanying cultural ideas would have developed within those boundaries; Anderson describes how the administrators created what amounted to nationalistic 'meaning' in their areas. That was necessary, of course; as Anderson says: 'In themselves, market-zones, 'natural'-geographic or politico-administratives, do not create attachments. Who would willingly die for Comecon or the EEC?'
In Europe, the boundaries of nation states as they emerged in the 18th and 19th centuries have got very little to do with the historical inter-play of noble families, and everything to do with the vernacular print-languages (it's almost possible to use the expression, 'cultures') which gained dominance, although this wasn't always along ethnic boundaries. In Ireland, for instance, (part of Britain at the time) English elbowed out Gaelic, and it was only much later that the Irish independence movement (like all such movements, closely associated with its own language) was able to hit back. Plenty more examples spring to mind, and the dissolution of the Austro-Hungarian Empire (which, amazingly, continued to use Latin as a state language until 1840) offers many of them.
With the exception only of some remaining 'primitive' societies, many of them in Africa, we all nowadays live in a nation state, which has a local monopoly of power and doesn't hesitate to use it to maintain its control over its citizens.
The growth of states and their powers on such a scale is a very recent phenomenon and owes much to technological development. It has taken place in the last 500 years, which is a bare 15% of recorded history, and a tiny fraction of the 50,000 or so years during which modern humans are thought to have occupied permanent settlements, requiring some type of hierarchical and/or administrative organization.
13 Anderson, B (1991) Imagined Communities: Reflections on the Origin and Spread of Nationalism, (2nd ed, first published 1983), Verso, London