Micah Zenko outlines the flaws in the decision to arm part of the Syrian opposition:

When an outside power openly backs certain rebel groups in a civil war, it immediately becomes invested in their prestige and power vis-à-vis other groups and in their success against the ruling regime. The outside power can fail: 1) If the groups receiving support see their relative power reduced, either through battlefield failures or political incompetence; 2) If the groups that the outside power hopes to marginalize actually gain prestige or power; or 3) If the ruling regime survives. Therefore, the United States and its partners are not merely “picking sides” in Syria, but picking sides of sides [bold mine-DL], and doing so with conflicting goals.

It’s foolish enough to pick the weaker side in a civil war as a proxy, but to pick the weakest part of the weaker side seems to be a guaranteed way to fail. Leaving aside the difficulty of identifying the “right” groups to arm, trying to find somebody worthy of being armed puts the cart before the horse. Instead of providing assistance to the groups most likely to succeed, the U.S. has chosen to arm that part of the opposition in the worst position to realize its goals of toppling Assad and “marginalizing” some large part of the opposition. Because the U.S. finds the strongest groups in the opposition to be unacceptable, it is committing itself to the part of the opposition that is most likely to be “marginalized.” There also seems to have been no thought given to the possibility that groups that accept U.S. assistance at this stage will be “marginalized” because the aid is coming from America. In the end, the effort to “marginalize” those parts of the opposition that the U.S. doesn’t support will mean stoking another conflict that would probably drag on for many more years.