China and NK consider themselves allies, as do the US and SK. And China+NK are not allied with US+SK. And that region is unlikely to be affected by any signific

China and NK consider themselves allies, as do the US and SK. And China+NK are not allied with US+SK. And that region is unlikely to be affected by any significant non-aligned players. [It's a bit more complicated than that, of course. And THAAD isn't 100% effective. But the details don't change the overall picture much.] If a missile is ever stopped by THAAD, it means SK+US didn't want it to land. Which probably means China+NK launched it, and did want it to land. Meanwhile, if SK+US decide to go on the offensive, THAAD won't stop their attack, but will mitigate any retaliation from China+NK. THAAD is bad for China+NK in both scenarios. In addition, since THAAD's existence makes it a whole lot less risky for US+SK to go on the offensive -- it makes it more likely that they would do so. And China+NK certainly wouldn't want to defend against a US+SK with THAAD being in place.