World War Three is generally thought of (Notably in Science-Fiction) as a Nuclear holocaust that would end up leaving the survivors in a primitive state of survival (Post-Apocalyptic World) hence the Fourth World War being Fought with sticks and stones following the destruction of the advanced technology & civilization by those same so-called advanced technologies & civilizations that are seemingly unable to stop destroying the world. The result of this process is to go back to the stage of the Stone Age so that the cycle of human folly can (most likely) repeat itself.
Once Albert Einstein was asked,"Can you say something about the Third World War?"
He replied:"No. I cannot say anything about the Third
World War, but I can say something about the fourth."
-“Two things are infinite: the universe and human stupidity; and I'm not sure about the universe.”
-"I know not with what weapons World War III will be fought with, but World War IV will be fought with sticks and stones."
-"I do not know how the Third World War will be fought, but I can tell you what they will use in the Fourth — rocks!"
-"Yes, I can say with absolute confidence about the fourth that it is not going to happen, because the third will be the last; it will finish all life on the earth."