First, who exactly would be armed? The perennial, deep problem of the Syrian opposition is that it remains fragmented, disorganized, and highly localized. This has not changed. The “Free Syrian Army” remains something of a fiction, a convenient mailbox for a diverse, unorganized collection of local fighting groups. Those groups have been trying to coordinate more effectively, no doubt, but they remain deeply divided. For all their protestations of solidarity, the Syrian National Council and the FSA show few signs of working well together, while repeated splits and conflicts have emerged in the media within the FSA. So to whom would these weapons be provided, exactly? I expect that what will happen is that foreign powers will rush to arm their own allies and proxies (or are already doing so); which ones are the United States meant to choose? While claims about the role of Salafi jihadists in the armed opposition are likely exaggerated, the reality is that we know very little about the identities, aspirations, or networks of the people who would be armed…
Fourth, how will Assad and his allies respond to the arming of the opposition? Perhaps they will immediately realize their imminent defeat and rush to make amends. But more likely, they will take this as license to escalate their attacks, to deploy an ever greater arsenal, and to discard whatever restraint they have thus far shown in order to stay below the threshold of international action. It would also be very difficult to stop Russia, Iran, or anyone else from supplying fresh arms and aid to Assad once the opposition’s backers are openly doing so. Providing arms to a relatively weak opposition will not necessarily close the military gap, then — it might simply push the same gap up to a higher level of militarized conflict.
Fifth, what will we do when the provision of weapons fails to solve the conflict? Arming the opposition is held out as an alternative to direct military intervention. When it fails to solve the crisis relatively quickly — and it most likely will fail — there will inevitably then be new calls to escalate Western military support to airstrikes in the Libya-style. In other words, what is presented as an alternative to military intervention is more likely to pave the way to such intervention once it fails.
Sixth, what if Assad does fall?