http://en.wikipedia.org/wiki/Doctrin...ear_Operations
The doctrine cites 8 reasons under which field commanders can ask for permission to use nuclear weapons:
An enemy using or threatening to use WMD against US, multinational, or alliance forces or civilian populations.
To prevent an imminent biological attack.
To attack enemy WMD or its deep hardened bunkers containing WMD that could be used to target US or its allies.
To stop potentially overwhelming conventional enemy forces.
To rapidly end a war on favorable US terms.
To make sure US and international operations are successful.
To show US intent and capability to use nuclear weapons to deter enemy from using WMDs.
To react to enemy-supplied WMD use by proxies against US and international forces or civilians