Final answer:
Austria-Hungary declared war on Serbia on July 28, 1914, after the assassination of Archduke Franz Ferdinand by a Serbian nationalist. Pressured by internal nationalist tensions and assured military support from Germany, Austria-Hungary sought to suppress Serbian nationalism, triggering a chain reaction of alliances that led to World War I.
Step-by-step explanation:
Austria-Hungary declared war on Serbia on July 28, 1914, a decision influenced by the assassination of Archduke Franz Ferdinand by a Serbian nationalist in Sarajevo. This event fueled an intense climate of nationalism and marked the inevitable ignition of World War I. The Austro-Hungarian ultimatum, prepared with the intention of being unacceptable, was nearly met by Serbia, excepting a few points which violated Serbian sovereignty. Emboldened by the unconditional military support from Germany, assured through the famous "blank check theory," Austria-Hungary saw an opportunity to challenge Serbia and quell the Slavic nationalist movements within its borders. The war declaration set off a cascade of mobilizations and alliances throughout Europe, and by early August, multiple great powers were at war, marking the onset of a global conflict.