System of alliances: The European powers were divided into two main alliances: the Triple Alliance (Germany, Austria-Hungary, and Italy) and the Triple Entente (France, Russia, and Britain). The assassination of Archduke Franz Ferdinand set in motion a chain of events that led to the activation of these alliances and the outbreak of war.
Austrian ultimatum to Serbia: After the assassination, Austria-Hungary issued an ultimatum to Serbia, which was seen as responsible for the attack. The ultimatum was deliberately designed to be unacceptable, and Serbia's partial acceptance led to Austria-Hungary declaring war on Serbia on July 28, 1914.
Russian mobilization and German declaration of war: Russia, as an ally of Serbia, began to mobilize its army. Germany, seeing this as a threat, issued an ultimatum to Russia to halt mobilization. When Russia refused, Germany declared war on Russia on August 1, 1914.
French and British declarations of war: France, an ally of Russia, and Britain, an ally of France and Belgium, subsequently declared war on Germany on August 3 and 4, respectively.
Global expansion of the conflict: The war spread beyond Europe as the alliance systems drew in other countries, including the Ottoman Empire, Bulgaria, Greece, Japan, Italy (which switched sides in 1915), Portugal, Romania, the United States (in 1917), and others.
Thus, what started as a localized conflict in the Balkans quickly escalated into a global war involving multiple countries and continents, becoming known as World War I.