The risk of sending any military support to the rebels is that it’s not clear how the U.S. could arm moderate rebels without some of those weapons ending up in the hands of extremist fighters. Groups like Jabhat al-Nusra, the increasingly successful Islamist rebels who are allied with al-Qaeda, would likely end up with some of the arms, whether small or heavy. And these weapons don’t disappear once the conflict ends, nor do they stop working if they’re transferred out of Syria into, say, Lebanon or Iraq.
So the case against sending small arms is two-fold: first, it’s unlikely to turn the tide against Assad’s forces, for the reasons Chivers explained above; second, extremists are bound to end up with some of those guns, which they could use to terrorize Syrian civilians or foreign targets. To be clear, the case against small arms is not necessarily a case for heavy weapons, which after all could also end up in the hands of extremists. But it’s easy to see why both advocates and critics of greater U.S. involvement are warning against sending small arms, which analysts such as the Brookings Institution’s Shadi Hamid have called a “half measure.”