Originally Posted by NorCal DC4

WW3.
Here's what I'd predict in terms of escalation:

1) Pakistan's nuclear weapon launch systems fall into the hands of fleeing Taliban.
Convinced that their final days are imminent...they decide to strike a death-blow against their perceived infidels... i.e. the Judeo-Christian alliance. Israel & the USA are key targets. Jerusalem is the first city struck, with attacks in major US cities to follow closely thereafter.

2) In retaliation, the United States launches against targets within Pakistan and along the Afghan Border. In our final hour, targets in Iran and Northern India are also struck.

3) If India or Iran has their nuclear program operational they too would likely target either the United States or it's allies & interests overseas.

Aftermath:
The intense concentration of radiation in the Northwestern Himalayan mountain range is spread eastward over the Tibetan Plateau, Mainland China, and eventually to the West Coast of the US.
Simlarly, the Persian Gulf is rendered uninhabitable by radiation blown inward from the Israeli coastline.
The major metropolitan centers of the United States are reduced to ash, or made unihabitable by the influx of radiation.

Within hours, roughly one-third of the world's population has died either in the nuclear strikes themselves, or of radiation poisoning.

Long-term:
Billions of cubic meters of ash and radioactive particulates are carried into the troposphere as a result of the massive explosions.
Within a week, nuclear winter ensues. Much-needed solar radiation dramatically declines due to the long-term effects of high-altitude particulates. Crops fail globally and the average temperature begins to fall. Even cities thousands of miles away from the nuclear strikes eventually fall victim to famine and disease.

Within 6 months to a year of the nuclear strikes, the human race is extinct.

Happy Apocalypse!