Final answer:
Gavrilo Princip assassinated Archduke Franz Ferdinand of Austria in June of 1914, sparking the onset of World War I.
Step-by-step explanation:
Gavrilo Princip assassinated Archduke Franz Ferdinand of Austria in June of 1914, which sparked the onset of World War I. Princip, a young Serbian nationalist, shot and killed the Archduke and his wife during their visit to Sarajevo. The assassination set off a chain of events that led to Austria's invasion of Serbia and the activation of the European alliance system, ultimately triggering the start of the war.