A couple of definitions pulled from the web:
A nation is a community of people formed on the basis of a common language, territory, ethnicity etc. A country may be an independent sovereign state or part of a larger state, a physical territory with a government, or a geographic region associated with sets of previously independent or differently associated people.
There is a difference between the terms nation, state, and country, even though the words are often used interchangeably.
Country and State are synonymous terms that both apply to self-governing political entities. A nation, however, is a group of people who share the same culture but do not have sovereignty.
A country is defined as a region or an area of a land which is controlled by its own government. The term ‘nation’ refers to a community or group of people who share the same history, language, descent and a common government.
Europe is broken down into a patchwork of language groups which does not fit neatly over areas defined by its major (generally Christian in origin) religions. Commonality of language and/or religion are frequently used by politicians to segment their territories.
Many of the European wars are fought between groups with subtle differences of culture, but headed by politicians/sovereigns who coveted something controlled by others. For example, Serbs and Croatians speak the same language (a Slavic dialect), but write it using two different alphabets and largely belong to two different religious sects (Serbs are Eastern Orthodox and use the Cyrillic alphabet, while Croats are Roman Catholic and write using the Roman alphabet). There are many similar examples.
The historical formation of many of our European nation/States is derived from the conflicts of post-Roman feudal principalities and kingdoms centuries ago where the local populations adopted the religion/language of their ruler. Frequently, the populations of adjacent (sometimes warring) States are very similar in cultural background and their “vast” differences are small sparks which have been fanned into flames by their leaders.
A current example is the parallel ethnic similarities between Russians (though, admittedly, Russia is far more diverse than being just Russian) and Ukrainians. The current conflict has gone a long way towards exacerbating the differences between the populations of the two States than any other action I can think of since Ukraine was separated from the Russian Empire at the end of the First World War.
The human associated social behavior of founding counties, I guess, is an extension of the survival trait that larger groups are more secure than smaller family units. That, however does not mean that the constituent parts of that entity have to be of a common background and humans have developed leadership techniques to address that, but the fault lines still exist and can be exploited to fracture the group - especially during times of stress. This has, in the past, affected the US (a country whose politicians are constantly trying to manipulate into being a nation) as well as other countries from time to time.
Jeff