Archduke Franz Ferdinand was assassinated in 1914 by Gavrilo Princip, a Bosnian Serb nationalist, sparking tensions that led to World War I. While his assassination was the immediate trigger, underlying political tensions, militarism, and alliances also played significant roles in causing the war. It's debatable whether the war could have been entirely avoided, given the complex web of geopolitical rivalries and tensions at the time. Diplomatic efforts might have mitigated the conflict, but the underlying causes made the outbreak of war highly likely.