Archduke Franz Ferdinand, heir presumptive to the Austro-Hungarian throne, was assassinated in Sarajevo on 28 June 1914. His death profoundly destabilized an already tense European diplomatic order. In the weeks that followed, Austria-Hungary, with backing from Germany, presented Serbia with a harsh ultimatum. Serbia accepted most but not all of the demands. On 28 July 1914, Austria-Hungary formally declared war on Serbia. That declaration set in motion a network of alliance obligations, mobilizations, and diplomatic failures that expanded a regional crisis into a continent-wide war. The declaration came amid rapid diplomatic and military preparations. Russia, seeing itself as protector of Slavic interests and Serbia in particular, began partial mobilization. Germany, allied with Austria-Hungary, demanded that Russia halt mobilization and issued an ultimatum to France, with which it feared encirclement. When diplomatic efforts faltered, Germany declared war on Russia on 1 August and on France on 3 August, and invaded Belgium on 4 August to implement the Schlieffen Plan. Britain, bound by treaty and public commitments to Belgian neutrality and alarmed by the prospect of German domination of the continent, declared war on Germany on 4 August. Within days most of Europe was at war. Historians emphasize that the assassination did not cause the war by itself but acted as the immediate trigger within a volatile system. Longstanding factors—militarism, imperial rivalries, entangled alliances, arms races, and nationalist tensions on the continent and in the Balkans—had created a situation in which a crisis could escalate rapidly. The July Crisis of 1914 involved intense diplomatic exchanges, hurried military timetables, miscommunications, and decisions shaped by domestic politics and fear of strategic disadvantage. The speed of escalation surprised many contemporaries. Leaders and publics initially expected a short, limited conflict; instead, the war evolved into a prolonged, industrial-scale conflict with unprecedented casualty levels. The declarations of war in late July and early August 1914 transformed Europe’s political geography, led to mobilization of millions of soldiers, and set in motion events—territorial changes, revolutions, and geopolitical realignments—that reshaped the 20th century. Scholars continue to debate how responsibility for the outbreak should be apportioned among the major powers. Questions persist about whether different decisions during the July Crisis—more patient diplomacy, delays in mobilization, or firmer restraint by any one government—might have averted or limited the war. What is clear in the historical record is that the assassination of Franz Ferdinand was the proximate event that precipitated a rapid series of state actions, culminating in the declarations of war that plunged Europe into World War I.