Ethnic groups, and culturally separate nations desiring autonomy, can form a foundation for separatist pressures. Regionalisation can be a useful safety valve, which might increase the acceptability and responsiveness of governance for distinct cultural groups. Regions can be granted autonomy in some decisions, whilst other functions of governance remain under central control.
The degree of autonomy offered can vary considerably. In a federal model, like that of the USA, separate States have their own legislatures and governors. The question of balance between the individual States and the federal government was carefully considered when the US Constitution was approved, but it has shifted somewhat over time. It was designed to distribute power, in the name of freedom and localisation. It wasn’t primarily driven by different cultural identities, but the comparative stability of that structure can suggest that it could also be used to divide a country along ethnic lines.
There are some aspects of governance which are particularly relevant to preserving cultural identity: for example, autonomy in the teaching of languages and history. Regionalisation has worked well in practice where there is a clear association between a cultural group and a particular geographic region; the Catalonian autonomy in Spain is an example of regionalisation to re-establish an older identity which pre-dated the unification of Spain, but separatist pressures remain, and at the time of writing are unresolved as the separatist’s leader went into exile – being described by the Economist as The man who wasn’t there.
Historic boundaries can cross into other countries. For example the Basque region overlaps both France and Spain, and there are Kurds in Turkey, Syria, Iraq and Iran. Unification of the Basques or the Kurds across international borders would be very difficult to achieve in practice, and granting them more autonomy within one country would put pressure on the neighbouring countries to do the same.
 The Federalist Papers are “a series of eighty-five letters written to newspapers in 1787-1788 by Alexander Hamilton, James Madison, and John Jay, urging ratification of the Constitution”, according to a brief summary published on History.com in 2009 which explained their purpose – to justify the division of power between the individual States and the federal government – and the context in which they were written. This was available in May 2018 at http://www.history.com/topics/federalist-papers.
The Gutenberg press publication of the collection in full was available then at http://www.gutenberg.org/ebooks/18.
 Michael Keating described the history of Spain’s “Minority Nations”, and the way in which their governance is evolving within the framework of Spain and Europe, in a paper entitled The Minority Nations of Spain and European Integration. A new framework for autonomy? This paper was available in May 2018 at http://cadmus.eui.eu/bitstream/handle/1814/6122/Keating_2000_MinoritynationsofSpain.pdf?sequence=3.