Which event sparked the first declaration of war in World War I on June 28, 1914?

The assassination of Archduke Franz Ferdinand of Austria-Hungary is widely recognized as the event that ignited the series of tensions and alliances leading to the outbreak of World War I. On June 28, 1914, Ferdinand was killed in Sarajevo by Gavrilo Princip, a member of a nationalist group seeking independence for the Slavic peoples under Austro-Hungarian rule. This assassination set off a chain reaction of political and military mobilizations among the major powers of Europe, as alliances were activated and nations began to declare war on each other in a rapid sequence.

Leading up to the assassination, Europe was already experiencing heightened nationalism, militarism, and a complex web of alliances that made tensions acute. The assassination of Ferdinand was the catalyst that caused these underlying stresses to erupt into open conflict. Austria-Hungary's subsequent declaration of war against Serbia marked the official beginning of the conflict that would escalate into a global war. The other options listed, such as the invention of the airplane, the signing of the Treaty of Versailles, and the start of the Industrial Revolution, do not pertain directly to the immediate causes of World War I, but rather to broader historical contexts or later developments in the case of the Treaty of Versailles, which was