Anyone who has taken a World History course at some point or another will probably be able to recall some idea of what nationalism was/is. In a nutshell, the term "nationalism" refers to strong feelings of patriotism and pride for one's home nation. It has played a large role in shaping history: it was nationalism that inspired William Wallace (a.k.a. Braveheart) and his followers to fight for Scotland's freedom; it was nationalism that led a fractured group of mediterranean states to unite to form Italy; it was an extreme nationalism that gave Hitler the notion of a worldwide German empire.
Nationalism is also apparent in music, especially music of the late Romantic Period. Ever since the days of the early Classical Period, music had been dominated by the "Austro-German tradition"... musical style, form, and language were dictacted by what was coming out of Austria and Germany. However, around the middle of the 19th century, composers from many "other" countries began trying to establish distinctive "national styles" for their own countries.
They did this using the dances, folk songs, history, and national legends of their countires as the basis for their works. For example, Antonin Dvorak made heavy use of folk dances from his native Bohemia (now called the Czech Republic). His harmony is also very reminiscent of Bohemian music, such as in his famous "From the New World" Symphony No. 9 or his Piano Quintet in A. In Russia, a group of composers nicknamed "The Mighty Five" worked to bring a national flavor to Russian music. Two of the more famous members of this group are Alexandr Borodin and Nicolai Rimsky-Korsakov.
Nationalism continued into the Modern
Period as well. Hungarians Béla Bartok and Zoltán
Kodály traveled the Hungarian countryside, collecting folk
melodies for use in their music. In America, composers like George
Gershwin, Scott Joplin, and Aaron Copland used American elements
such as African-American spirituals, jazz, and ragtime in their
compositions. Additionally, Brazil gained musical representation
Villa-Lobos and his nationalistic writing.