What was the immediate cause of the outbreak of war?

The immediate cause of the outbreak of war in World War I was the assassination of Archduke Franz Ferdinand of Austria-Hungary on June 28, 1914, by Gavrilo Princip, a Bosnian Serb nationalist. This event triggered a series of diplomatic crises and alliances, leading Austria-Hungary to declare war on Serbia. The intricate web of alliances and mutual defense agreements quickly escalated the conflict, drawing in major powers across Europe and igniting a widespread war.