What was one of the causes of the increasing european tension prior to world war i?

Serbia.

Serbia was essentially one of the causes of the tension just before World War I, because Russia wanted to keep Serbia under its protection because of the large Slavic population in Serbia, while Austria-Hungary believed that they had the right to take over Serbia. With the assassination of the leaders of Austria-Hungary, Archduke Ferdinand and Sophie by a Serb, it lead to the start of World War I.

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