What had to happen to start WW1?

World War I was triggered by a series of complex factors, but the immediate catalyst was the assassination of Archduke Franz Ferdinand of Austria-Hungary on June 28, 1914, by Gavrilo Princip, a Bosnian Serb nationalist. This event set off a chain reaction of alliances and militaristic posturing among the major European powers. Austria-Hungary declared war on Serbia, drawing in allied nations and escalating the conflict into a full-scale war. Nationalism, Imperialism, and militarism also contributed to the tensions that led to the outbreak of the war.