The assassination of Archduke Franz Ferdinand of Austria in June 1914 is widely recognized as the catalyst for World War I. The event set off a chain reaction among the major European powers, invoking a series of alliances and entanglements. Austria-Hungary declared war on Serbia, prompting Russia to mobilize in defense of Serbia. This quickly escalated into a wider conflict that involved multiple nations due to existing alliances. The war ultimately resulted in significant geopolitical changes and led to the Treaty of Versailles, shaping the course of the 20th century, including the rise of totalitarian regimes and the conditions that contributed to World War II.