

To add to this, N Korea also has a huge conventional army, and is a very mountainous country. Lots of soldiers+mountains=very bloody to invade.
This is also why Iran is fairly safe from ground invasion. It’s like a gigantic Switzerland, which if you’re familiar with WW2 history, even Hitler left Switzerland alone despite kinda wanting to occupy the place. The cost was just too high compared to the benefits, so, y’know, may as well skip it and invade the USSR instead.
Modern war planners mostly know better than to count on everything going well.