Rebel/Sunni with MANPAD
After the fall of Ghaddafi arms flooded out of Libya and were used by rebels as diversely located as Mali and Sinai.
In Syria the Rebels have captured these MANPADs when they have overrun government bases. What is to prevent these weapons being smuggled out of Syria into the West to be used at a civil airport near you?
And if we provide the extra MANPADs and ATGMs the Rebels are asking for doesn't that just increase the probability of weapons being smuggled out? An ATGM could make a mess of a high value city centre building.