Which event is considered the start of World War I?

The assassination of Archduke Franz Ferdinand of Austria is widely recognized as the event that sparked World War I. On June 28, 1914, Ferdinand was killed in Sarajevo by a Bosnian Serb nationalist, which set off a chain reaction of political and military alliances being activated. This assassination created significant tension between Austria-Hungary and Serbia and ultimately led Austria-Hungary to declare war on Serbia. The complex web of alliances involving major European powers quickly escalated the conflict, resulting in a general war.

The other events listed, while historically significant, do not mark the beginning of World War I. The sinking of the Titanic occurred in 1912 and was a major maritime disaster but had no direct connection to the causes of the war. The invasion of Poland happened much later, in 1939, marking the start of World War II. The signing of the Treaty of Versailles in 1919 concluded World War I but did not initiate it. Thus, the assassination of Archduke Franz Ferdinand is clearly identified as the catalyst for World War I.