North Korea is on the verge of getting nuclear weapons that can reach Japan, the US, and much of the rest of the world, despite repeated pledges not to. Sanctions have proven ineffective, and the US has no capability of cutting off Korean trade with China. Options are few and grim:
(1)An all-out surprise attack on N. Korea's nuclear and missile facilities and military. This would undoubtedly provoke a violent response, which possibly could include nuclear weapons reaching South Korea and possibly Japan. On the positive side, it might encourage Iran to strictly adhere to its treaty with the US. The reaction of China and Russia is unpredictable.
(2)An ultimatum to N. Korea, threatening utter destruction. This would be impossible unless China and Russia agree.
(3)Magically assassinate Kim. Problem: magic likely required. Even if successful, results completely unpredictable.