North Korea restarted its nuclear program roughly after the US had invaded Iraq, because they knew they could be next. After the US / NATO bombed Libya, North Korea's leadership was probably certain they were next if they didn't have nuclear weapons. I'm not saying that it's great North Korea had developed its nukes, but at the same time they did it for good reason. The Americans can blame their own disastrous foreign policy for that. Now, there is a reason why nuclear-armed states do not fight each other, and if there is a potential for conflict, they prefer to talk. That's because the alternative is much worse and there can be no winners. North Koreans know that too, and I doubt they'd be willing to start a war they can't win. Therefore, it's naive to claim that they will become "immune to diplomacy". Of all things, diplomacy becomes the only thing available to influence policy when the alternative is nuclear weapons.