Just wondering. I'm posting his here instead of post-1900 because there's a good chance you could get a World War pre-1900 going. In fact some argue that the Napoleonic Wars should be considered the first World War, but we'll leave that aside for now.
What's the earliest possible timeframe to have a World War on? We'll define "world war" as a war involving campaigns on multiple, disperate continents, with multiple theatres across distances of thousands of miles. Alexander's wars of conquest don't count, for example, because although he campaigned across Greece (Europe), the Near East (Asia), and Egypt (Africa), all the action was still confined to a single broad area.
On the other hand, I suppose you could count everything the Mongolians did. Even though it was mostly in Asia.
Anyway. Gogogo!