Modern history witnessed two world war's. These wars were fought for land and power. Since every major country is nuclear power now so, i hope there can't be any war over land as no one can afford a nuclear catastrophe . So, what can be the scene from where third world war will start.
As it is said "water is life" so this third and ultimate war for humanity will start over trivial fights for life which mean water.Green house effect is fastly causing global warming which in result will increase the melting of glaciers . This in turn will increase water flow in rivers for some years but after this rivers will start drying up. Right now many countries are sharing water through several international treaties like indo pak water treaty. Countries in the upstream will start violating the treaty to supply water to their own population. This will create a situation where countries in downstream will be forced to start a war. Even inside a country there will be civil war between states for water. Unlike other world war's,This war won't have a two sides.It will be a collection of several inter country and intra country wars..

This is just a rough idea of my story on third world war.