Well, we would be able to do it, but we wouldn't. For one too many innocent lives would be affected. Something like a stray tank round or missile strike that kills ten or twenty civilians is easier to keep getting out of the news, a nuclear strike isn't. We wouldn't do it, not because of the innocent blood on our hands, but because of the fallout from the UN or from our own people. Secondly, if trace amounts of radiation arrived here from the Japan reactor, imagine how much fallout would arrive here from even one of our lower yield nukes? (Many times the power of the A-Bombs we gave to Japan.) Besides, if we wanted to level the country, why drop expensive nuclear weapons that pose a large risk to our safety, when we could use conventional bombs and missiles which are cheaper, and can do just as much damage in a large strike. Maybe more if done strategically. Finally if we did drop a nuke, Kim Jong-Un would probably just eat it.