In regards to nuclear warfare, the goal isn't to survive - its to have he capability o utterly destroy your enemies in retaliation so they never use their nukes.
And Russia and PRC would want to bring down the US with them if they think the US is attacking them.
It takes a few minutes for nukes to go across the globe. When one nation starts seeing nukes coming their way, it is highly unlikely that they will wait to see if those nukes are aimed at them or some other nation. And one nation can say they are targeting a different nation, but if they lie the nation that gets hit can't respond.
Which is why nuclear warfare is based around pre-emptive strikes and automatic retaliatory strikes. Nations aren't going to wait to see if a nuke actually hits them before they respond. They will assume that the nukes are after them, and then retaliate while they can to damage the first nation as much as they can.
Which is why nuclear weapons have only been used for warfare once in the history of this planet, and why nuclear war has never been actually fought on this planet - because those are weapons that are so destructive that they cannot be survived, and so opponents try to ensure the destruction of their enemies instead.
I know that NK is no threat the the US, because if it was then our government would have spent a TRILLION dollars to stop them from getting the bomb instead of on killing tent dwellers in the middle east.
So far, I see exactly two people on this thread who have a clue.
I have to wonder if it has something to do with having grown up during the Cold War, doing Civil Defense drills in school, all in deadly earnest.
There are complexities involved that most people don't seem to want to consider.
Complexities that may well mean we CAN'T just nuke NK flat in response to a single nuke-tipped missle thrown at a US target... or an SK or Japanese target, etc.
There's the threat of what a single nuke could do used in High-Altitute EMP mode to the entire North American continent.
There's China, and it's proximity to NK, and it's own nuclear arsenal. Not to mention SK and Japan could be downwind.
There's the question of Kim Jong Un, who grew up in NK knowing he was going to succeed his father as dictator, and how much contact there is between the world between his ears and Planet Consensus Reality... and whether he believes NK could get away with it.
There's also the question of how much does NK do that doesn't have tacit Chinese approval. Many people who study these things carefully believe China uses NK as a stalking horse and Judas Goat, much like Iran/Syria/Lebanon. A client state.
There's the question of what would happen to SK, with Seoul (capital city, over 40% of the population) sitting within range of NK's massive conventional artillery poised on the DMZ.
It's not as simon simple as some wish to believe, and there are reasons why massive US nuclear retaliation is NOT an automatic and assured response... and that very lack of certainty is one of the things that makes NK potentially dangerous.