The assassination set off a chain of events that led to the outbreak of war. Austria-Hungary, with the backing of Germany, issued an ultimatum to Serbia, demanding that it punish those responsible for the assassination and take measures to prevent such acts in the future. Serbia agreed to most of the demands, but some were seen as unacceptable, leading Austria-Hungary to declare war on Serbia on July 28, 1914.
Russia, as a Slavic nation and ally of Serbia, began mobilizing its army in support of Serbia. Germany, viewing this as a threat, issued an ultimatum to Russia to cease mobilization. When Russia refused, Germany declared war on Russia on August 1, 1914. France, as Russia's ally, then declared war on Germany on August 3, 1914.
Thus, the assassination of Archduke Franz Ferdinand by Gavrilo Princip can be seen as the event that triggered the outbreak of World War 1.