Well the fear is that it alllows Russia and Iran to gain a bigger foothold in the Middle East.... Really this was F'd up by Obama, Saudi, and Israel funding/supporting ISIS (aka Syrian Rebels) to try to topple the Regime. Since we did that Assad asked Russia to help and they did... Now Russia and Iran (which also was aiding them) stand to gain ground while we lose face if we withdraw... All Obama's fault for getting us there in the first place. Israel doesn't want us to leave so they have an excuse to attack Syria more. (If you look into it... it's highly probable they were involved in the Arab Spring too....)