Well, the most important effect of nationalism in Europe in 1900s was the World Wars. Citizens of many countries were unhappy with the economy and the way things were in their states. This, along to the critical approach towards other nations and the growing national socialism led to the outbreak of military conflicts, pogroms, and eventually the holocaust and facism during the World War II. One important thing is that were talking here about very extreme nationalism.
Austria feared increasing rebellion among its subjects. ;
In the early 1900s, nationalism in Europe fueled political tensions that contributed to the outbreak of World War I. It encouraged militarism, imperial competition, and the independence movements of various ethnic groups, ultimately culminating in a conflict triggered by the assassination of Archduke Franz Ferdinand. This intense national pride and desire for self-determination significantly reshaped the political landscape of Europe.
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