Just wanted to note something here.
A nuke does not have to land on a major population centre in order to be devastating. It doesn't need to travel even 200 miles. It doesn't ever have to even enter US airspace in order to do massive damage. All it has to do is go up. The most strategically sound use of nuclear weapons on any heavily tech-based nation like the US is to employ the electromagnetic pulse effect of a nuke. When a nuke is detonated in the upper atmosphere, it's blast energy cannot be readily converted into kinetic energy since there is nothing really around it, so it remains as it started; an EMP. The reasons for an EMP blast effect are many. Firstly, the "kill zone" it has is huge beyond anything at ground level. A single medium nuke would have a kill zone roughly comparable to the radius of north america (actually it's slightly larger, maybe 5-10%). Secondly - it doesn't have near the amount of radiation damage to the soil. Employing a nuke in the high atmosphere scatters the radiation, and doesn't leave a lingering wasteland that exists for thousands of years. Third - it doesn't destroy agricultural improvements, leaves the land itself mostly intact; all it destroys is technology. Consider the results if a nuke was fired from a pleasure yacht a short ways off of the shore of the united states on both sides. In the united states, every bit of technology that utilizes electricity will fry. Probably most of canada as well. That means no factorys, no clocks, no heaters, no communications equipment, no planes (Air force jets included), no cars (excepting deisel engines), no refrigerators, etc... Basically, the country would fall back into a technology level roughly comparable to 1700. All the sophistication of our military would collapse, and we wouldn't have enough food to feed our citizens. People would die all over the place, deprived of needed food, pure drinking water, and heat. Plauges would break out, as all the medical technology would be gone and people would be concentrating in unsanitary enviroments in order to survive. Probably the country which fired the nukes would even help with disease spreading.

Nukes are a big threat... I think it's a good thing that the leaders of the country are afraid of them.