By the last quarter of the 19th century, nationalism lost its liberal-democratic character and became a narrow, aggressive creed. Nationalist groups grew intolerant of each other and readily went to war, while major European powers manipulated nationalist aspirations to further their own imperialist aims.
The most serious source of tension was the Balkans — a region of ethnic and geographical variation (Romania, Bulgaria, Serbia, Bosnia-Herzegovina, etc.) largely under the declining Ottoman Empire. The spread of romantic nationalism and the disintegration of the Ottoman Empire made the region explosive. Each Balkan state wanted more territory at others' expense.
This was worsened by big power rivalry: Russia, Germany, England, and Austro-Hungary each tried to counter others' hold over the Balkans and extend their own control. Their rivalries — over trade, colonies, and military might — led to a series of Balkan wars and, ultimately, the First World War (1914).
Source: Chapter 1 — Nationalism in Europe, Section 6: Nationalism and Imperialism
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