Increased Serbian Nationalism led to the incitement of conflict
Serbian Nationalism directly led to the Assassination of Archduke Ferdinand of Austria-Hungary by a Bosnian Serb, Gavrilo Princip. Even preceding the actual assassination, Serbia had been seen as a rising threat to Austria. Austrian annexation of Bosnia and Serbia’s strong emergence following the Balkan Wars all resulted in creating a threat towards Austria-Hungary. Additionally, there was consistent rebellion by Serbs against Austria in an effort to bolster their own national identity, even further creating volatility. The growth of nationalistic groups such as 'Unification or Death' ('Black Hand') etc. also led to further initiatives to separate from Austria.
Serbian nationalism is not responsible for World War 1. Austrian and German response to Serbian nationalism resulted in the conflict arising and sucking other world powers into the conflict due to allyships.