Final answer:
The main reason for the outbreak of World War I was the assassination of Archduke Franz Ferdinand.
Step-by-step explanation:
The main reason for the outbreak of World War I in 1914 was the assassination of Archduke Franz Ferdinand. On June 28, 1914, Gavrilo Princip, a young Serbian nationalist, shot and killed Ferdinand and his wife. This act triggered a chain of events, leading to Austria-Hungary declaring war on Serbia, which in turn triggered Russia, Germany, and other nations to join the conflict.