Simplest explanation is best.

Jong Un sees that he got Trump to drop precondition with nothing in return.
Now he wants to see if he can get Trump to stop military training with South Korea over threat of ending summit before it starts.

— lawhawk (@lawhawk) May 15, 2018

And Trump undermined our national security in the first place by elevating Jong Un to dictate terms here all while dropping precondition that North Korea give up its nuclear program before direct talks.

— lawhawk (@lawhawk) May 15, 2018

These are the people Trump listens to. They’re busy telling him what he wants to hear.

They’re the ones who are in his head and making him play by the numbers.