No China, on your plate?
...perhaps a mid-power vs. large power nuclear war
and I just added for clarification:
(without an opposing large power responding with nukes).
A China scenario is very scary for the US, even without the US becoming heavily involved in the fighting. Not only might the world economy collapse in a fashion to make 2008 look "insignificant", a lot of radiation could drift our way.*
*Even today, in most bodies of water in the US not contaminated by a local source, most of the mercury in our fish is coming from... China. (EPA won't come out publicly and say that directly, but the data to deduce this is right on their website.)