Nostradamus wrote thousands of verses, each of which can be interpreted in many ways. He's bound to get something right, some time, and we conveniently ignore the huge majority of times that he's way off.
To me it seems like there is not enough genuine competition between world powers for a world war to start. Compare the multiple European-based empires pre-WWI, or the much more balanced state of nations in the leadup to WWII. Now, the US is so conventionally dominant that no traditional world war can really take place, IMO.