I still dont see why people refer to Trump as some negotiating savant. Time after time, its been shown that hes notoriously thin skinned and easy to rattle. During the 3rd debate he was not doing terrible until Hillary commented that he's only successful because daddy gave him money. Trump's inherent identity and belief that he was responsible for all of his own success being under attack in a public forum caused him to become apoplectic. You could see the anger rise, his face turned beet red, he saw red, wasn't thinking clearly and it was then that he went off the rails and his performance suffered. It was clear he had been rattled and veered far off message.

I imagine this is his personality. Whatever the case, it has been a constant since I became aware of him from Howard Stern during his playboy years in the 1980's. New Yorkers are well aware of this, his ego and his compulsive need to settle scores with reporters and others for any perceived slight whether imagined or real when he was a fixture of Page 6. Someone like Putin or anyone else with a poker face and on even footing could play him like a fiddle, alternating between awe/flattery and hurling passive aggressive insults.

An NPR politics podcast had mentioned that Trump always responds to the last statement from the other side, even if it's irrelevant to the rest of the statement. Clinton leveraged that during the debates.

I can imagine foreign diplomats quickly catching onto this and using it to their advantage in discussions.