Final answer:
The assassination of Franz Ferdinand triggered the start of World War I, as it led to Austria-Hungary invading Serbia and activated the alliance system between various countries.
Step-by-step explanation:
The assassination of Franz Ferdinand triggered the start of World War I. On June 28, 1914, Gavrilo Princip, a young Serbian nationalist, assassinated Archduke Franz Ferdinand and his wife. This event led Austria-Hungary to make demands for restitution from Serbia, which Serbia refused, resulting in Austria-Hungary invading Serbia. The alliance system then came into play, with Russia supporting Serbia, France supporting Russia, and Great Britain supporting France, ultimately leading to the outbreak of World War I.