Final answer:
On June 28, 1914, Archduke Franz Ferdinand of Austria-Hungary was assassinated by Gavrilo Princip, a Serbian nationalist. This event precipitated Austria-Hungary's declaration of war on Serbia, setting off a chain of alliances leading to World War I.
Step-by-step explanation:
The assassination that sparked the outbreak of war in Sarajevo, Bosnia, was the killing of Austro-Hungarian Archduke Franz Ferdinand. This significant event occurred on June 28, 1914, when Gavrilo Princip, a member of the Serbian nationalist group, the Black Hand, fatally shot the archduke and his wife.
The assassination led to a series of demands for restitution from the Serbian government by Austria-Hungary. Serbia's refusal to meet all the demands resulted in Austria-Hungary declaring war on Serbia, thereby activating a chain of alliances that escalated into World War I. The underlying tensions that facilitated the start of the war included nationalism, militarism, imperial ambitions, and complex alliances. The death of Franz Ferdinand and the subsequent diplomatic fallout triggered what would become a global conflict, marking the beginning of the First World War.