Syrian President Bashar Al-Assad's sudden willingness to put its chemical weapons under international -- i.e. Russian -- supervision might allow the regime to avoid a US attack while at the same time preserving its advantage in Syria's civil war. Despite tough rhetoric from the United States, if an agreement can be reached on the modalities of safeguarding the chemical weapons, the US can avoid becoming embroiled in the Syrian conflict while still, more or less, saving face.
President Barack Obama's delineation of the "red line" that would trigger intervention if crossed trapped the US into acting in Syria. Not to do so would have been a serious blow to the US’ pre-eminent standing in global affairs.