What year did World War I begin?

World War I started on July 28, 1914, when Austria-Hungary declared war on Serbia following the assassination of Archduke Franz Ferdinand. Following this declaration, Germany's support of Austria-Hungary prompted Russia, Great Britain, France and Belgium to enter the war as allies of Serbia.

A Serbian nationalist assassinated Ferdinand on June 28, 1914, setting off a chain of events leading to the outbreak of war. Austria-Hungary refused to declare war until it had support from Germany, even though the country knew that Germany's support meant Russia, France and Great Britain were likely to also enter the war due to various political and military pacts.