The conflict that escalated into World War I was the assassination of Archduke Franz Ferdinand, which triggered a series of geopolitical tensions and alliances. This assassination on June 28, 1914, set off a chain reaction involving various nations. Following the murder, Austria-Hungary declared war on Serbia, leading to the activation of alliances: Russia mobilized to support Serbia, Germany declared war on Russia, and soon France and Britain were drawn in. These escalations turned a regional conflict into a full-scale global war, involving many of the world's major powers and resulting in significant casualties and political changes.