If the Great War of 1914 was a conflict between mainly European nations, why would historians refer to it as a world war?  How was the rest of the world affected?

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.

What war are you specifically talking about?