"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones."
Albert Einstien
People are task specific now and have no clue how to build a cell phone or a computer or even make metal. Once the flow of goods stops, look around and see what you could make again once it is destroyed. Once the power goes off and the flow of water stops, you can count with your fingers the days it will take for humans to revert to animals.
It has been said that a CME will hit the Earth in 2012 shutting off (whatever side of the world it hits) the power grid killing 2/3 of the human population. When that happens, there will be no World War around that time period. Every country would be forced to work together to save the part of the world that was effected by the CME. Now I'v notice this has gone off topic so to finish this, this is what the chain of events would be:
1: CME hits the Earth's Magnetic Bubble
2: The Radiation left from the CME will cause massive power surges, destroying the power stations
3: Due to the lack of power, no running water, no gas stations, generators will die out
4: Due to no water or lack of gas, people will die from illnesses, dehydration, famine
5: That part of the world would be in a dark age until new power stations are up and running, in which would take years since the major citys need to come first (e.g New York, Boston, Las Vegas)