"This is incoherent: in order to be in a position to threaten the US with nuclear weapons you must already be a nuclear power."
Not true. NK is stating their ambition to take hostile actions, this is the definition of threat, whether or not they have the capability. If intimidations towards America are the purpose of developing weapons, these are still threats. Arguably, NK has the nuclear capability but not yet the ability to launch the weapon to mainland US.
Hypothetically, if I state my hostile intention, right now, to shoot you in the head in a message over the internet, but I don't yet have the gun, am I still threatening? If I say, send me $1,000 or I'll poison your family's water source, but I don't have any poison, is this still a threat?
US doesn't act as if it holds a de facto veto over nuclear ambitions. Did Pakistan or India threaten its way to being a nuclear power? Did UK or France threaten its way? NK openly threatens the US with its nuclear potential. Iran has been an issue because it threatens the destruction of US and its allies.
If The US allows NK to be a nuclear power, will China accept Taiwan as a nuclear state?
Saddam Hussein massacred its own people, was a reckless leader, and fired ballistic missiles at US allies. He should have been removed with force earlier than 2003, and WMD justification wasn't needed for his removal.
Gaddafi, after giving up nuclear ambitions, openly threatened the US financial system. No government who cannot control its own people should have nuclear capabilities, and the Libyan conflict proved this. His removal was a highly successful operation, and Obama's feeling of regret is what made the situation worse because he disengaged. Nearly half a million Syrian civilians would have been saved if the Libyan-Gaddafi model was applied to Assad in Syria.
What about the counter examples of nations who didn't pursue nuclear weapons but decided to cooperate with US militarily and economically? Germany -- one of the best economies in the world, ranked 4th, with enormous potential in its future. Japan and South Korea, two economies ranked in the top 10 in the world, with excellent programs in medicine and technology.
There is a long list of countries who choose to not pursue nuclear capabilities, their governments are safe, their economies are strong, and their people have a high standard of living. This isn't about US dictating to the world, it's about doing what's right for the global economy and increasing the pace of technological progress.
What has North Korea done for its people? Why doesn't it have an economy? Why does it threaten half the world?
As I've explained above, NK surrendering its nuclear arsenal isn't tantamount to suicide. Iran still exists, has received billions of dollars, and has increased its power over the Middle East.
As a regular citizen without access to all information, I don't know how devastating it would be to use force against NK's nuclear threats.