How did nationalism threaten the existing political order of the 19th century

I will take an an example Austria-Hungary. Around 1880 it was a multi-cultural empire that included Bosnia, which was a Slavic nation, unlike Austria and Hungary. The feelings of nationalism in Bosnia (by other Slavs, who wanted Bosnians to join them) was what threatened the existing political order because it was what caused the assassination of the Austrian Crown Prince and eventually the World War.