The immediate cause of World War I that made the aforementioned items come into play (alliances, imperialism, militarism, nationalism) was the assassination of Archduke Franz Ferdinand of Austria-Hungary. In June 1914, a Serbian-nationalist terrorist group called the Black Hand sent groups to assassinate the Archduke. Hope this helps, plus this was the main reason why it happened plus if you see a link other people offer for the answer don’t put it in the web, I tested it out I got a virus. Anyways hope this helps!
