In nation-state is a single territory (generally contiguous) governed by a single government in which the vast majority of residents share a national (and often ethnic) identity called a "nation". The term "state" refers to the government and the term "nation" refers to the group of people who share the same identity. The experience of Bosnia in the breakup of Yugoslavia and World War II have taught us how maliable this sense of nationhood can be in times of crisis.
Cosmopolitan and Supernational Ethnic Identification
From the fall of the Roman empire in 532 C.E. to the 18th century, the notion of the "nation-state" was less established than it is today. People owed allegiance to an individual feudal lord whose territory was often a patchwork of non-contiguous fiefs cobbled together with marriage alliances and wars.
Towards the end of the 18th century and into the 19th and 20th centuries, the ideal of "nationalism", i.e. that your identity was significantly a function of being part of a large ethnic group called a "nation" which shared a contiguous "nation-state" which existed independent of whoever the current ruler was gained strength. It has also spawned many wars as groups who view themselves as "nations" look at the boundaries of existing "states" as illegitimate and try to form their own sovereign nation.
This outlook of the legitimacy of states has played a part in World War I (the Treaty of Versailles the ended the war recognized this principle), World War II (Hitler sought to unify the German nation), the Bosnian Civil War following the break up of Yugoslavia in the wake of the break up of the Soviet Union, the Chechen War in Russia following the break up of the Soviet Union, the Troubles in Northern Ireland, the Basque terrorist movements in Spain, the Israeli-Palestinian conflict, the violence surrounding the partition of India and many other wars of this era.
It is not obvious this nationalism will hold sway forever, despite the arguments of scholars like Francis Fukayama that we are approaching the End of History. One way this era could end is a shifting in allegiances from a "national" identity, to a supranational or cosmopolitian identity.
For example, the European Union is helping many younger Europeans to identify first not as "Germans" or Frenchmen" or "Poles", but as "Europeans." Likewise, many members of the educated, liberal elites of Africa have, since the 1950s, viewed themselves as part of a "Pan-African" primary identity, rather than as member of a particular, arbitrarily created colonial legacy state, or as a member of a far more tiny tribe, clan, or ethnic group which is often found only in a small part of their state, and may also be found in parts of other African states. This is evidenced by growing interest in creating a strengthend African Union.
Supports of a nationalist view of identity would simply argue that these development are simply disputes over whether a nation is larger or smaller. But, the logical extension of a "regional identity" such as an identity as a "European" or an "African" is a "Cosmopolitan" identity, as a citizen of the world. The United Nations is the political embodiment of this idea, and is, quite frankly anemic. But, it is never the less symbolic of this hope, and many people who work within the UN itself and related international agencies, share this identity.
Some on the left and the right who oppose globalism, fear the cultural damage that could be done if some form of Cosmopolitan identity, fueled by global corporate capitalism, destroys local identities and cultures as they homogenize into a single world culture as the world grows smaller through improved communications and transporation technologies.