Final answer:
Austria-Hungary wanted to suppress Serbian ambitions to unite the South Slavs, which threatened Austro-Hungarian territorial integrity. The assassination of Archduke Franz Ferdinand became the pretext for the ultimatum, and with Germany's unconditional support, Austria-Hungary felt emboldened to confront Serbia, leading to World War I.
Step-by-step explanation:
Austria-Hungary created an ultimatum for Serbia primarily to assert its dominance and crush Serbian independence, as Serbia posed a threat to the stability of the Austro-Hungarian Empire. This was due to Serbia's ambition to unify the Slavic people, including those under Austrian rule in Bosnia, into one Serbian-led nation. The assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, by a Bosnian Serb was the immediate cause that Austria-Hungary used to put forth the ultimatum. Serbia's refusal to comply with all demands and the backing of Austria-Hungary by Germany with a "blank check" of support ignited the First World War, transforming a regional conflict into a global war.