There can be no morally credible argument against intervention - by someone. Leaving Syria to descend into the kind of sectarian violence that devastated neighboring Lebanon in the 1980s would condemn hundreds of thousands to premature, violent death. Syria is five times the size of Lebanon. The risks of leaving it to degenerate into a failed state are surely higher than the risks of intervention.
But why should it be the United States that once again attempts to play the part of global cop?