Preconditions are like granting a veto to the extremists.  No government is monolithic. In every regime there will be moderate forces willing to use diplomacy and negotiations and extremist elements that do not. By demanding that preconditions be fulfilled before a meeting, all power is given to the extremists. Israel-Palestine is an example of this. By often demanding that violence cease before negotiations can take place, the US and Israel give a veto over negotiations to Hamas, who are the perpetrators of violence. They get to decide if and when negotiations take place. Instead, the president of the United States should always try to engage and strengthen moderates within these regimes.