Final answer:
The assassination of Franz Ferdinand activated a series of alliance commitments and escalatory moves among European powers, quickly inflating a Balkan crisis into a world war by August 1914.
Step-by-step explanation:
The assassination of Archduke Franz Ferdinand is widely recognized as the immediate catalyst that sparked the outbreak of World War I. On June 28, 1914, the heir to the Austro-Hungarian throne was killed by Gavrilo Princip, a Serbian nationalist.
This event set off a chain reaction within the complex system of alliances in Europe. Austria-Hungary, seeking to punish Serbia, presented an ultimatum which Serbia partially rejected. With the backing of Germany, Austria-Hungary declared war on Serbia, activating alliance commitments throughout Europe.
Russia mobilized to defend Serbia, leading Germany to declare war on Russia. France, allied with Russia, was then pulled into the conflict, and Germany's invasion of Belgium brought Great Britain into the war. These events all pivoted on the initial spark provided by the assassination, and within a few months, a localized European conflict escalated into a larger world war.
In summary, the assassination of Franz Ferdinand was the spark in an already powder-keg situation of nationalism, imperialist ambitions, and a tangled web of alliances and treaties. The desire to show strength, honor alliances, and preemptively strike against perceived threats caused a rapid escalation from assassination to global warfare. By the end of August 1914, World War I was fully underway, with major powers entangled in a conflict that would ravage Europe.