Which event is commonly known as the spark that ignited World War I?

The assassination of Archduke Franz Ferdinand is widely recognized as the critical event that ignited World War I. This incident occurred on June 28, 1914, when the heir to the Austro-Hungarian throne was shot by Gavrilo Princip, a member of a nationalist group seeking independence for Slavic peoples from Austro-Hungarian rule. This assassination set off a chain reaction of alliances and mobilizations among the major European powers, leading to the declaration of war.

The significance of this event lies in the existing political tensions and complex alliances in Europe at the time. The assassination led Austria-Hungary to issue an ultimatum to Serbia, which, upon being partially rejected, resulted in Austria-Hungary declaring war on Serbia. This act triggered a series of mobilizations and declarations of war among many nations due to their alliances, ultimately plunging the entire continent into conflict.

In contrast, while the sinking of the Lusitania did fuel public sentiment against Germany and contributed to the United States' eventual involvement in the war, it did not directly trigger the outbreak of hostilities in 1914. The invasion of Poland occurred much later, in 1939, marking the beginning of World War II, not World War I.