Answer:
The event that most historians agree started World War I was the assassination of Archduke Franz Ferdinand of Austria-Hungary. The Archduke was assassinated on June 28, 1914, in Sarajevo, Bosnia, by a Serbian nationalist, Gavrilo Princip. This event set off a chain reaction of alliances and mobilizations among the major powers of Europe, leading to the outbreak of war in August 1914.The assassination of the Archduke was the immediate cause of the war, but the underlying tensions and rivalries among the European powers, including competition for colonial territories, arms races, and shifting alliances, had been building for many years before the assassination.
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