Which event triggered the start of World War I?

The assassination of Archduke Franz Ferdinand is widely recognized as the catalyst for the start of World War I. On June 28, 1914, the heir to the Austro-Hungarian throne and his wife were murdered by Gavrilo Princip, a member of a nationalist group called the Black Hand. This event set off a chain reaction of alliances and political tensions that had been building in Europe for decades.

Following the assassination, Austria-Hungary issued an ultimatum to Serbia, where the assassin originated. When Serbia's responses did not satisfy Austria-Hungary's demands, it declared war on Serbia on July 28, 1914. This declaration triggered a complex web of alliances that resulted in multiple countries entering the conflict, escalating it into a full-scale war that involved nations from around the globe.

While the sinking of the Lusitania, the signing of the Treaty of Versailles, and the invasion of Poland each played critical roles in different historical contexts, they are either events that occurred later in the timeline related to World War I or related to its aftermath and subsequent conflicts, rather than the immediate cause of the war itself.