“Can they just keep threatening us (and its neighbors)..... ?”
It’s what they do.
I dated a South Korean for a while. When I asked about some rattling NK threats being made at the time, she rolled her eyes at me and asked why I listen to anything NK says.
NK thrives on belligerence. It’s the only way they can psychologically justify the brutality imposed on the populace, and be taken anything close to seriously by anyone else. If NK stops making hyperbolic threats, then it’s just a failed state.
Seems like the Iranian do likewise.
The problem is that once it gets loud enough, they are not taken seriously. With our porous border, I wouldn’t doubt that there will be an unexpected attack on the US.