Experienced weapons inspectors like David Kay say that it is untenable to move the vast store of chemical weapons out of Syria. If an agreement is reached, they will have to be protected while inventoried and later destroyed within Syria by an outer ring of Syrian security forces supplemented by some UN troops. Assad can only keep his promise as long as he is in power. He must stay in power as long as it takes -- and in the US we have been dismantling our chemical weapons since the Nixon era.
So, ensuring the removal of chemical weapons from Syria is to ensure the survival of Assad.
Welcome to the inescapable complexity of the Middle East.