A third world war is hardly necessary...
On a social level I completely agree with you, all this crap about fighting wars for tracts of land is stupid. Just look at what is happening in the Middle East right now, or what has repeatedly happened in Kashmir. Even my country (Ireland) was embroiled in conflict in the North for decades. It is stupid and we don't need it.
But there is one very good strategic reason for a third world war; oil. We will probably be able to convert civilian oil use over to renewable sources, but you will not be able to convert the military. You can't have an electric-powered tank, or an ICBM that runs on wind power.
The absurd thing is that we will probably end up fighting a war in order to control a resource that we only really need in case we have to fight a war.