"But insisting on preconditions – or preconcessions – for negotiations along with escalating military pressure is likely to accelerate us down a path toward war, without having truly tested what could be achieved through talks.
"So here's the stark choice for our next president, made even starker by the increasing nuclear and missile threat from North Korea: Do we, or do we not, sit down with the North Koreans in direct formal talks, before this crisis escalates even further?"
So that is the point. Preconditions are settlement barriers. No one really knows what will happen in a mediation but insisting on a precondition prevents you even from finding out. It prevents the other side from understanding and addressing your concerns even before you begin. It defeats the exact purpose of mediating: communication. Insisting on preconditions is a little kid covering his ears and saying nyah,nyah, I can't hear you.
A journey of a thousand miles begins with the first step. You don't get to start in the middle. Sometimes that first step is the very hardest. You won't know if you can cover a vast distance unless and until you take that first step. Don't use preconditions to avoid solving a dispute. At least take that first step and see if you're going in the right direction.