The significance of the assassination of Archduke Franz Ferdinand in starting World War I lies in its role as the immediate catalyst that triggered a complex web of alliances and conflicts. His assassination on June 28, 1914, by a nationalist led Austria-Hungary to declare war on Serbia. This event activated a series of treaties and mobilizations among major powers, escalating into a full-scale war. The aftermath reshaped international relations and set the stage for significant political and social changes, ultimately leading to the conclusion of monarchies and the rise of new ideologies.