That's easy.
Iran launches a nuke at Isreal.
Isreal responds by counter nuking Iran and Lebannon (where Hamas is).
The rest of the Islamic countries will then be embroiled in a war with each other as some will come to Isreals defense and be deemed traitors to Allah by the Fanaticals.
The US, thinking that Iran has giving nuclear material to Al Queda, bombs the Pakistani border with Afghanistan and then invades.
India, thinking this is a good time to reclaim Kashmir, launches nukes at Pakistan.
Pakistan, in retaliation for the US bombing and invading, launches missiles at Afghanistan and India.
North Korea, seeing the US distracted, invades South Korea and launches it's missles at the West coast of the United States.
The missles, being poorly constructed and designed, veer off into China and piss off the Chinese. China invades North Korea and nukes Tibet as an afterthought.
Russia, seeing China distracted, launches missiles at China.
Venezuala attacks it's neighbors and embroils that continent in an all out war.
Al Queda around the world then starts to attack oil facilities world wide thinking that if those are destroyed then the West's military machines will fail within a year.
The US will open up ANWR and other areas formally protected from oil drilling pissing off the Green Movement and starting a civil war in the US.
Mexico, seeing the US distracted will invade to take back all the territory the us "stole" from them.
After all is said and done, it's the Canadians that will win.
But that will be short lived because of the nuclear winter and 200 years later yellowstone AND Mt Rainier will erupt further plunging the world into an ice age which will wipe out all life on the surface of the planet and about 2/3 of the oceans life.
The only things that will be left alive will be what ever is living next to the geo-thermal gyesers on the ocean floor.
Cheers,
Sklar