If it was literally any other President, who had just met with China, they would know.
But we have Trump.
And he has no idea.
The best case scenario, is North Korea prepares a "launch", the US strikes what turns out to be a fake and there's no significant fallout, literally or figuratively. Kim Jong Dumbass gets emasculated on global television and his staff forces him to listen to reason.
Worst case, they basically respond to Trump's threats by rolling down the window and tossing a nuke into South Korea. At that point, it doesn't matter what happens, the result is a loss for the world as a whole. (They're not making it to Japan, the sea is thick with ships)