Never mind the legalities of the situation. Never mind morality either. Just answer the pragmatic question: Is it ever a good idea to start a nuclear war? Because that's the notion that U.S. President Donald Trump is actually playing with. He didn't say exactly that, of course. He said that "If China is not going to solve (the nuclear threat from) North Korea, we will. That is all I am telling you." But in the context of that interview with the Financial Times, it was clear what he meant.
Trump was saying that if China did not use the tools at its disposal (political influence, trade sanctions, withholding financial aid) to make North Korea give up its nuclear weapons and long-range rockets, then the United States would use the tools at its disposal to accomplish the same goal.
This does not necessarily mean that the U.S. would launch a large nuclear attack against North Korea. If you are really serious about carrying out a "disarming strike" that destroys all of North Korea's nukes, you probably should do exactly that. (You never get a second chance to go first.) But maybe the U.S. Air Force would promise that "precision" non-nuclear weapons could accomplish that goal, and maybe some gullible people would believe it.