Which day did World War I begin?

The story behind the answer

World War I began on July 28, 1914, when Austria-Hungary declared war on Serbia. This followed the assassination of Archduke Franz Ferdinand on June 28, 1914, in Sarajevo. The declaration triggered a cascade of alliance commitments—the “July Crisis”—as Russia mobilized to back Serbia, Germany supported Austria-Hungary, and France and Britain were drawn in, transforming a regional conflict into a global war.

Memory tip: think “28–28”—assassination on June 28, war begins exactly one month later on July 28. June lit the fuse; July ignited the war.