One of the main reasons Austria-Hungary declared war on Serbia was the assassination of Archduke Franz Ferdinand on June 28, 1914. The Archduke, who was the heir to the Austro-Hungarian throne, was killed by Gavrilo Princip, a member of a nationalist group called the Black Hand, which sought to promote Slavic nationalism and independence from Austro-Hungarian rule.
Austria-Hungary viewed the assassination as a direct challenge to its authority and stability, especially given the rising nationalist sentiments among ethnic groups within its multi-ethnic empire. In a context of rising tensions and a desire to assert its power in the Balkans, Austria-Hungary issued an ultimatum to Serbia containing demands that were intentionally harsh and difficult for Serbia to accept. When Serbia responded to the ultimatum with some concessions but rejected certain key points, Austria-Hungary declared war on Serbia on July 28, 1914, setting in motion a chain of events that ultimately led to the outbreak of World War I.
1 answer