Two?? In addition to the USA and Russia, the UK, France, China, India and Pakistan also have nuclear weapons. It's universally believed that Israel does too, although they've never admitted it and all the world's spies have not been able to prove it. North Korea has tested one but probably does not have any ready to deploy. That genie is out of the bottle and so far he's been doing a great job. No nation has attacked a country with nuclear weapons, although paramilitary non-national groups like Al Qaeda have. And to try to put that into perspective without disrespecting the victims of 9/11 and their grieving families, in the first decade of this century terrorists have killed almost exactly the same number of Americans as peanut allergies.
I think you mean to say "no state," and not "no nation." Plenty of groups which claim to represent nations have attacked nuclear powers - Al Qaeda is one such, and likewise Israel is frequently attacked by such groups. But even then, the correct statement is not "attacked a country with nuclear weapons" but rather something like "waged a frontal, undeniable attack on a country with nuclear weapons." That doesn't rule out deniable attacks-by-proxy, which are fairly routine. Although, even that would have some qualifiers considering US attacks on Pakistan, etc. But overall, yeah, a nuclear deterrent is just that.
I don't think nations would respond with nuclear warfare. If one of the smaller countries such as North Korea or Iran used nuclear weapons on another I believe the best and most likely response would be to build a massive coalition of Nations and invade the agressor nation in the first two weeks. There are very few major powers out there who would be okay with any nuclear strike against a civilian population. I would assume even China would turn against North Korea if it attacked another nation. Maybe something similar to Watchmen. If a terrorist group were to strike with nuclear weapons I would assume a similar all out war would be conducted against them. Just as long as something similar to the movie Southland Tales doesn't happen. Generally I would think a nuclear attack would unify the world. I feel the best reason to retain some nuclear stockpile is to fight an alien attack fleet. I think I will enjoy the worlds move into cyber-warfare, it seems like so much more fun, it makes it easy for anyone to basically attack any other nation and get away with it.
That's what i was going to say, but I tried to take the poster seriously - only that proved to be a misstep because the OP has only this one post and has never returned. I think better to just ignore this thread from now on.