Final answer:
Austria-Hungary declared war on Serbia in response to the assassination of Archduke Franz Ferdinand.
Step-by-step explanation:
Austria-Hungary declared war on Serbia as a response to the assassination of Archduke Franz Ferdinand of Austria-Hungary by a Serbian nationalist. The assassination was seen as a direct challenge to the authority and prestige of the Austro-Hungarian Empire. Austria-Hungary issued a set of ultimatums to Serbia, which were rejected, leading to the declaration of war.
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