There is a very strong parallel between Nixon/N Vietnamese and Trump/N Korean negotiations. NV knew Nixon was in trouble back at home, the peace movement was strong, and the next congress would almost certainly be dominated by antiwar and anti Nixon Dims. All they had to do was string the talks out a few months and the Dims would sell out NV's enemies. In the end Nixon got the US out of Vietnam and turned the war over to the friendly allies in VN, Cambodia, and Laos; congress then cut off all funding so there was no chance of their survival.
Congress is undermining all Trump's foreign negotiations in precisely the same way. It was likely no coincidence at all that the Cohen hearing and exposure coincided with the Trump/Kim talks. Why come to an agreement with someone you think won't last, when the other guys will offer a better deal?