I've yelled at people so many times when bringing up "World War III". The name simply wouldn't make any sense historically speaking...
The Great War and World War II were European conflicts. Since they owned most of world anyway the rest of the world was also involved, but the two wars were European Wars simply on a larger scale.
The reason why World War II is called World War II is because the major players were all on the same teams. Germany vs. Russia and also England vs. Germany. These were the three main countries in those wars ideologically and militarily.
It should also be added the Second World War was only twenty years after the first one. Twenty years ago Home Alone came out, it's really not that long ago.
As others have pointed out, the Cold War was an extention of the unsettled business of the Second World War. If anything is called World War III it should be the Cold War.
A World War today would have no connection to the first two. An example however, see Punic Wars, the third one, although fifty years later involved the same conflict as the first two. Also see Crusades.
In addition, technology has grown to such an amazing degree that a full large scale war is no longer realistic. If you have a thousand tanks and I have one nuke I am militarily superior to you. Nuclear war is also not possible, only nuclear massacre. If you look at standing armies your living in the wrong century. This may have made you a powerful nation a hundred years ago but not today. Today economic might has long replaced military power.
