What is the best example of nationalism causing tensions in Europe?

One of the best examples of nationalism causing tensions in Europe is the First World War. In the years leading up to the war, many European nations experienced a surge of nationalist sentiment, particularly in the Balkans. This led to increased tensions between different ethnic groups and calls for independence, particularly from the Austro-Hungarian Empire. The assassination of Archduke Franz Ferdinand of Austria in 1914, by a member of a nationalist group, sparked a chain reaction of events that ultimately led to the outbreak of war. Nationalism played a significant role in the causes of the First World War and in the tensions between European nations.