World War I was triggered by the assassination of Archduke Franz Ferdinand of Austria-Hungary on June 28, 1914, by a Bosnian Serb nationalist. This event set off a chain reaction of alliances and mobilizations: Austria-Hungary declared war on Serbia, Russia allied with Serbia and mobilized, Germany allied with Austria-Hungary and declared war on Russia and France, and Britain declared war on Germany after its invasion of Belgium. The conflict quickly expanded into a global war involving many nations.
In summary, while the immediate "start" can be linked to the assassination of Archduke Franz Ferdinand, the broader causes involved a complex web of political alliances, militarism, imperial rivalries, and nationalist tensions across Europe.
