Franz Ferdinad got shot (Hungary) by some other dude and everyone allied with Hungary wanted to gang up on the dude that shot Franz and then everyone allied with the dude's country then had to fight Hungary and also everyone allied with Hungary. Yay for alliances!
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.
