World War I erupted in the summer of 1914, ignited by a complex web of factors. The assassination of Archduke Franz Ferdinand of Austria-Hungary in Sarajevo was the spark, but underlying tensions had been simmering for years. A system of entangling alliances obligated nations to defend one another, transforming a regional conflict into a global war. Nationalism fueled aggressive ambitions, and a widespread belief in the swiftness and decisiveness of war blinded leaders to its potential horrors. The failure of diplomacy and a series of miscalculations ultimately led to the outbreak of a conflict that would reshape the world.
