Should there be a WW III, then nukes will be used and let's say the U.S and most of Europe, most of Asia, and parts of Africa would be blown to smithereens. Fallout from the nuclear war would kill kill all others who survive. WWIII would be the end of the world.
Nuclear would be my main worry and it would certainly turn nuclear very quickly. It is scary to think that two or three high powered nuclear weapons could wipe out an entire country. I actually do not think anyone would survive if the world started using nukes against each other, the radiation levels would kill everyone.