Final answer:
The assassination of Franz Ferdinand sparked World War I. Gavrillo Princip shot and killed the Archduke of Austria-Hungary and his wife, which led to a chain reaction of declarations of war.
Step-by-step explanation:
The event that sparked the entire world-changing conflict of World War I was the assassination of Franz Ferdinand. On June 28, 1914, Gavrillo Princip, a young Serbian nationalist, shot and killed the Archduke of Austria-Hungary and his wife. This event ignited a chain reaction of declarations of war, starting with Austria-Hungary declaring war on Serbia and triggering Russia, Germany, and France to all enter the war.