Final answer:
The assassination of Archduke Franz Ferdinand of Austria-Hungary by Gavrilo Princip on June 28, 1914, led directly to the outbreak of World War I, as the ensuing diplomatic crisis escalated into full-scale war due to the complex system of alliances between European powers.
Step-by-step explanation:
The immediate cause of World War I was the assassination of Archduke Franz Ferdinand of Austria-Hungary. On June 28, 1914, in Sarajevo, Gavrilo Princip, a member of the Serbian nationalist group known as the Black Hand, fatally shot the Archduke and his wife, Sophie. This event is commonly referred to as the 'spark' that ignited the war's outbreak, propelling the major powers of Europe into a series of declarations of war due to a complex network of alliances. Austria-Hungary's demands to Serbia after the assassination, and the resulting refusal by Serbia, led to Austria-Hungary's decision to invade. Supported by Germany through alliance obligations, Austria-Hungary's aggression caused Russia to mobilize forces in support of Serbia, which in turn led to Germany's declaration of war on Russia and France. World War I had thus begun, encompassing nations globally in a conflict that lasted until 1918.