Short answers, pulled from the story.
Nationalism is an ideology or movement holding that the nation should be congruent with the state. It maintains that a nation should govern itself free from outside interference, that the nation is the natural basis for a polity, and that it is the only rightful source of political power.
In the English language, nationalism dates to around 1798, and the term gained wider prominence in the 19th century. Its connotations became increasingly negative after 1914. The Prussian scholar Johann Gottfried Herder originated a related sense of the term in 1772 in his Treatise on the Origin of Language.
Ethnic and civic nationalism are the two main divergent forms, with the distinction popularized by Hans Kohn in the 1950s. Kohn described civic nationalism as Western and more democratic and ethnic nationalism as Eastern and undemocratic, though since the 1980s scholars have pointed out flaws in this rigid division.
Benedict Anderson, in his 1983 book Imagined Communities, argued that nations are socially constructed communities imagined by people who perceive themselves as part of a group. He tied the rise of nationalism to print capitalism and the proliferation of printed media in vernacular languages.
Nationalism has featured in movements for freedom and justice, has been associated with cultural revivals, and encourages pride in national achievements. It has also been used to legitimize racial, ethnic, and religious divisions, suppress minorities, undermine human rights, and start wars, being frequently cited as a cause of both world wars.
During the 19th century nationalism became one of the most significant political and social forces in history and is typically listed among the top causes of World War I. It drove Italian unification, or the Risorgimento, which formed the Kingdom of Italy in 1861, and German unification achieved by Otto von Bismarck after 1871.