Negative Security Assurance
In order to provide incentive for non-proliferation, the nuclear power states offered a negative security assurance, that as a matter of treaty they would not use nuclear weapons on non-nuclear signatories of the NPT (with some fuzziness, like allies of nuclear powers could be attacked [ie a Soviet armoured division crossing Poland was not safe even if Poland was a NPT signatory], and more recently, bio and chem weapons might receive nuclear retaliation).
By threatening pre-emptive nuclear strikes on an NPT signatory, not as retaliation for an attack violating the treaty, the US has effectively destroyed negative security assurances (and this was not an empty promise, the US refrained from nukes in Korea and Vietnam, the French did not use nukes, nor did the Russians in Afghanistan or the Brits against Argentina).
So, why would anyone now honour the Non-Proliferation Treaty?
It seems clear that with the new policy, the only "safe" course is for each nation to get a deterrent quickly. This is the first lesson of the post "axis-of-evil" world.
And there are 10-20 nations who could so so very quickly, and another 10-20 who could with a very major effort.
And, while there is some marginally stable equilibrium for bilateral mutual assured destruction between sane players, finding stable equilibra for multiplayed MAD scenarios is hard. Impossible if one or more player is not rational.
I do not want to see nuclear weapons used in anger on the planet I live on.