More than half the world is already in drought. You'd be better of spending you trillions on desalination plants than on a World War. Politicians are just much more smarter than that. Also if we were to cut green house gas emissions fast enough than erratic whether would reverse itself and drought would cease to be a problem. It just so happens that is what politicians have been occupying themselves with for the past 5 years.
As for oil. The world has learned from the US mistake of a grab for natural resources. Its been deemed a failure and I doubt very much that any nation would be dumb enough to make the same mistake. Plus like i said before the world is moving in a completely different direction. That is, towards alternative renewable energy.
Precious metals. If there was to ever be a world war driven by the need for Gold, Silver, etc. than that would have happened a long time ago. Its just not enough to enter a devastating war that you could lose. Especially with the risk of nuclear warfare that has grown so vastly in the past 50 years.
In my opinion it would take something very big to trigger it. I'm not sure what will trigger it. But the main players will start it off. Those are the countries who bare nuclear arms. A prediction would be India, Israel & America (allies will follow) VS. Iran & Russia (allies will follow) for example.