Which event triggered the start of World War I?

The assassination of Archduke Franz Ferdinand is recognized as the event that triggered the start of World War I. This event, which occurred on June 28, 1914, in Sarajevo, was pivotal because it set off a chain reaction of alliances and mobilizations across Europe. The assassination was carried out by Gavrilo Princip, a member of the nationalist group known as the Black Hand, which sought to end Austro-Hungarian rule in Bosnia and Herzegovina.

Following the assassination, Austria-Hungary issued an ultimatum to Serbia, where the assassin had ties. The tensions escalated quickly, resulting in Austria-Hungary declaring war on Serbia on July 28, 1914. This declaration activated a series of alliances and military agreements involving major European powers, leading to a wide-scale conflict.

The other options, while significant in the context of World War I, did not trigger its start. The sinking of the Lusitania, for example, fueled anti-German sentiment in the United States but occurred later, in 1915. The invasion of Belgium by Germany in August 1914 was an immediate consequence of the war's declaration, but it was initiated after the assassination and did not serve as the trigger. Similarly,