Why were austria and serbia enemies in world war 1?

Austria and Serbia were enemies in World War I primarily due to nationalistic tensions and territorial disputes in the Balkans. Serbia sought to expand its influence and unite South Slavic peoples, which threatened Austria-Hungary's stability, as it contained various ethnic groups, including Slavs. The assassination of Archduke Franz Ferdinand of Austria by a Bosnian Serb nationalist in 1914 further escalated these tensions, leading Austria-Hungary to issue an ultimatum to Serbia. This ultimately contributed to the outbreak of the war as alliances were activated and conflicts escalated.