Why NK can act only rationally, while US could choose to act irrationally? Both sides can act irrationally, as history shows.

Why would it be irrational for the US to attack (a non nuclear-ready) NK? If NK didn't have nuclear weapons, invading it would be an option for the US- as they did recently with Iraq, Afghanistan, etc. I'm not sure what the gains would be, but it's not completely irrational to start a war when you have such overwhelming military superiority.

It is 'more rational' for an American president to avoid war with NK and leave it as something for his successors to deal with. Kicking the can down the road is easier than a war where potentially a few million South Koreans get killed in the crossfire.

This is, after all, what American presidents have been doing for the past few decades.

Same reason why the Korean War ended. China wouldn’t like a US-friendly country so close to its borders. NK is a great buffer zone and a great way to keep US distracted over South Korea. If NK gets overtaken it means Korea would extend all the way to china’s borders which they would not accept.

It depends on circumstances. Right now:

1. NK is not expansionary.

2. NK has nukes. US does not want anyone to use nukes, even a little one.

3. Any attack on NK means many deaths in SK.

4. Bad press.