What was the state that was a thorn in Austria-Hungary's side and a primary cause of World War I?

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Serbia was the state that was a thorn in Austria-Hungary's side and the primary cause of World War I. The assassination of Archduke Franz Ferdinand of Austria-Hungary by a Serbian nationalist was the spark that ignited the war. Austria-Hungary declared war on Serbia, leading to a domino effect of other countries joining the conflict.

What Was the State That Was a Thorn in Austria-Hungary's