Nations are political entities. A Nation consists of one or more Regions under the same government.
Regions can be joined together physically, by trimming and binding with cables and chains. After a couple of years, they begin to grow together. By five years, the joint is stable, and not easily broken again.
There are may ways for a Region to be joined to a Nation. A Region may be empty, and claimed by any nation. A Region may be taken by conquest, and joined to another nation by force. An independent National Region may also voluntarily unite with another Nation for many regions - commonality of culture, protection from duress, strong economic ties, and others.
Nations may be made up of any number of regions. To a certain extent, the bigger they are, the safer they are, but big Nations made up of many Regions are like machines with many moving parts. The more parts there are, the higher the chance that one may fail.
Regional Cultures may take much longer to grow together than the physical skylands they are made of. Some never do grow together - sometimes Regions revolt and attempt to separate themselves from their constituent Nations. Other times civil wars can wrack a Nation in two - physically and spiritually.
Thus many Nations prefer their constituent regions to be uniform in culture if possible. This can merely mean they refuse Regions with different cultures admission to their Nation. Other possibilities are cultural pressures to conform, breaking up the troublesome culture with forced resettlement in small pockets among the main culture, even genocide in extreme cases.
Some Nations, however, welcome the diversity of many cultures. Making it work is never easy, but can be accomplished with determination and tolerance.These virtues, of course, have never been distributed in over-abundance among humans, so such places are rare among Nations.