--United Kingdom: Why would our biggest ally blow us up?
--France: See United Kingdom
There's something wrong there. 
NATO, American Revolution
The most likely is highly unlikely, and that would be Israel.
You seriously think Israel is more likely to launch nukes at the US than Russia?
As has been said, the only answer to this question is Russia because of the phrase "full scale". Of course the odds of that are astronomically low currently, but that can change in fairly short order. It's probably slightly more likely for us to get involved in a nuclear
exchange with the PRC, but I'd be shocked if any of their targets would be US cities.
Poll
