When state leaders negotiate one on one, there's a certain amount of bravado, of machismo (or female equivalent), of sizing each other up, of seeing how big a backbone (to use a surrogate term) the other leader has, etc. Leaders around the world are thinking all they gotta do is push Trump's buttons. Or, alternatively, flatter him.
In a real crisis, what's Trump going to do? Whine on Twitter what a big meany Kim or Putin or Assad or [fill in the blank] is?