What event sparked the start of World War I?

The assassination of Archduke Franz Ferdinand of Austria by a Bosnian Serb nationalist on June 28, 1914, is recognized as the primary catalyst that ignited World War I. This pivotal event exacerbated existing tensions among the major European powers, which were entangled in a complex web of alliances and rivalries. Following the assassination, Austria-Hungary issued an ultimatum to Serbia, and when Serbia's response was deemed unsatisfactory, Austria-Hungary declared war. This act set off a chain reaction of mobilizations and declarations of war among the allied nations, ultimately leading to a full-scale global conflict.

The other events mentioned, while significant in their own contexts, occurred either after the war had already begun or were not direct causes of the war's outbreak. For instance, the naval blockade by Britain and the sinking of the Lusitania were significant in the context of U.S. involvement in the war later on, while Germany's invasion of Poland was a trigger for World War II, not World War I. Thus, the assassination ultimately stands out as the most direct and impactful event that led to the outbreak of World War I.