>I don't recall us bombing...China

As far as i know, USA was seriously considering doing this in 1964. However the soviets said that would mean war, so they didn't.

North Korea is so firmly in China and Russia's sphere of influence that bombing them would be hard, especially given how much conventional weapons they have pointed at south korea. That said, would you really want to be north korea? economic warfare has done a number on them. in many ways they are the poster child for nuke != winning.

Israel probably snuck through by doing it early enough and secretively enough that it was fait accompli. They probably tested their nukes before the nuclear non proliferation treaty was even signed.

so really you have india and pakistan. The fact there are so few exceptions kind of proves the system works.

> Nuclear nonproliferation relies on active enforcement.

It relies on getting nukes being an irrational move for most countries. Yes part of that is your enemies will start preventive wars to stop that. A very major part of it is the economic consequences of developing nukes is usually not worth it. Part of it is the smart strategy is to just do most of the work and stop before getting nukes - if shit hits the fan you can get nukes quickly, but going up to the line without crossing it has none of the negative consequences.