The beginning of World War I was triggered by the assassination of Archduke Franz Ferdinand of Austria-Hungary on June 28, 1914. This event in Sarajevo set off a chain reaction of alliances and tensions that quickly escalated into a full-blown conflict. Austria-Hungary issued an ultimatum to Serbia, which resulted in mobilizations and declarations of war due to entangled alliances involving major powers. The assassination exemplified the deep-rooted nationalist sentiments and geopolitical tensions in Europe at the time. As a result, the war began in August 1914, leading to four years of extensive fighting and significant global changes.