I already have my own theory on what will start the third world war, and that's natural resources (either petrol or food supply)
There will come a point where, rather than enforcing more stringent breeding policies, countries will choose to fight over land instead due to a twisted sense of entitlement and patriotism. I honestly would not be surprised if it's either China or India who begin the war, since they're not only nuclear-armed, but also countries with ballooning populations.
Currently playing: Uncharted: Drake's Fortune, NBA2k11, Metal Gear Solid, Picross 3d