Describe the influence of nationalism in the nineteenth-century.

Nationalism was hugely influential in Europe during the nineteenth-century. The French Revolution of the previous century had unleashed a radical ideology which held that the ultimate sovereign power resided with the nation rather than in a monarch. This idea proved inspirational to millions across the length and breadth of Europe living under kings and queens, many of whom were of a completely different nationality to those they ruled. The Austro-Hungarian empire, for example, was made up of many different nationalities including Germans, Czechs, Slovaks, and Hungarians. Distinct national identities began to emerge, united by common bonds of language and culture. It's no accident that the age of nationalism saw the emergence of a variety of national literatures which had previously been suppressed or marginalized.
Nationalism reached its apogee in the revolutions of 1848-49. Although these national uprisings petered out, the spirit of nationalism lived on. Where initially nationalism had been a liberal, emancipatory ideology, as the century wore on, it became decidedly more reactionary and exclusionary. The main reason for this development is that the political and social elite, who'd traditionally felt threatened by the rise of nationalism, successfully managed to incorporate it into their worldview, using nationalism as a means of consolidating their hold on power.
The unification of Germany is a prime example of this. The old Prussian aristocracy, the Junkers, were profoundly hostile to nationalism when it first emerged, seeing it as a threat to traditional society, not least because of its liberal and democratic character. Yet as time went on, and under the leadership of Bismarck, nationalism provided a useful instrument in forging a united Germany, one that would ensure the continued dominance and control of the social elite and a powerful Prussian army which served their interests. In other words, what started out as a movement from the bottom up became more top-down as the nineteenth-century progressed.