Perhaps I missed it; has North Korea done anything past the ceasefire they signed more than 50 years ago? Because, you know, that pesky state of war existing and stuff, not sure it would really much matter if there were a ring of ships and an embargo going on - if they shot at the ships, they’d be breaking the same ceasefire agreement they’ve broken 97 times.
They were conventional operations whose effect were limited to local theater, psychologically terrorizing S. Korea and making their regime look good by showing that N. Korea can stand up to U.S..
Now N. Korea is close to(or already has) long-range nuclear missiles and a high-yield nuclear bomb(probably a hydrogen bomb. This puts the whole thing in an entirely different perspective.
They are crossing the absolute red line and think that they can still get their way by resorting to their old well-honed script of threats accompanied by diplomatic maneurvers.
It is one thing to be shot at with a bullet from a handgun once in a while which you can duck and dodge (and may shoot hack a bullet or two), but entirely another thing to be aimed at by anti-tank missiles.