Short answers, pulled from the story.
Benedict Anderson defined a nation as "an imagined political community." He called it imagined because members of even the smallest nation will never know, meet, or hear of most of their fellow-members, yet each carries an image of their communion.
An ethnic nation, exemplified by the German peoples and linked to philosophers like Johann Gottlieb Fichte, is built on shared language, religion, culture, history, and ethnic origins. A civic nation, traced to the French Revolution and the thought of Ernest Renan, is centred on a willingness to live together and results from an act of affirmation.
Nationalism, the belief that state and nation should align as a nation state, did not become a prominent ideology until the end of the 18th century. The scholarly consensus holds that nations are socially constructed and a distinctly modern phenomenon.
The word nation entered Middle English around 1300 as nacioun, from Old French nacion and Latin natio, meaning birth, origin, breed, or race of people. It traces back to the Latin verb nasci, to be born, and the Proto-Indo-European root gene-, meaning to give birth or beget.
Philip S. Gorski argued the first modern nation-state was the Dutch Republic, built on political nationalism rooted in biblical nationalism. Liah Greenfeld instead held that nationalism was invented in England by 1600, calling England the first nation in the world.
Samuel P. Huntington argued that in the post-Cold War world, cultural and religious identities would be the primary source of conflict. He set out the theory in a 1992 lecture and a 1993 Foreign Affairs article titled "The Clash of Civilizations?", then expanded it in a 1996 book, written in response to Francis Fukuyama's 1992 book The End of History and the Last Man.