What was one of the immediate causes of World War I?

The assassination of Archduke Franz Ferdinand is widely regarded as one of the immediate causes of World War I because it served as the catalyst that triggered the existing tensions among European powers. Archduke Franz Ferdinand was the heir to the Austro-Hungarian Empire, and his assassination on June 28, 1914, by a Serbian nationalist in Sarajevo set off a chain reaction of events. Following the assassination, Austria-Hungary issued an ultimatum to Serbia, which, when not fully accepted, led Austria-Hungary to declare war on Serbia. This declaration activated a series of alliances and treaties that drew multiple countries into the conflict, effectively transforming a regional dispute into a world war.

The other events mentioned, while significant in the broader context of World War I and subsequent geopolitical changes, occurred later or as consequences of the war rather than serving as immediate causes. The sinking of the Lusitania, for instance, contributed to changing public opinion and U.S. involvement in the war, but it occurred in 1915, after the conflict had already begun. The overthrow of the Russian Tsar and the rise of communism in Russia were significant developments during the war, but they were not immediate causes of its outbreak in 1914.