Rachel Kleinfeld starts her argument for arming Syrian rebels with the least credible reason:
Why arm Syrian rebels? Let’s start with Iran. The faster Syrian dictator Bashar Assad falls, the faster Iran loses its closest ally in the region and its main conduit for shipping weapons to terrorist groups that attack Israel and other U.S. allies. A Syria without Assad will further isolate Iran and could help force it to the nuclear negotiating table.
It’s a mistake to assume that taking more aggressive measures towards an Iranian ally will make it more accommodating on the nuclear issue. If another government were responsible for helping to topple an American client, Washington would not be more inclined to cooperate with it or yield to its demands. To the extent that Iran would be more isolated after the fall of Assad, there’s no reason to expect that to have an effect on its willingness to make a deal. If anything, losing an ally will strengthen the hard-liners’ hand inside Iran, since they will plausibly be able to portray U.S. policy in the region as a single-minded campaign to weaken and harm their government. It is doubtful that Israel is eager to have advanced weapons in the hands of the Syrian opposition, which is no more friendly to their interests than Assad. Prolonging and intensifying Syria’s conflict in the name of possibly undermining Iranian influence would be extremely short-sighted, since we have no way of knowing what new threats to the region might be created that are potentially more dangerous to all of Syria’s neighbors than Hizbullah is to Israel. If this is the “serious” interest-based argument for meddling in Syria’s war, it’s no wonder that it has been ignored.
Less credible still is the idea that America somehow “owns” the conflict in Syria. That’s not true, and the U.S. shouldn’t be dragged deeper into it because someone claims that it is. If the government begins directly backing armed factions in Syria’s conflict, then Washington will own their actions and be responsible for them. What begins as arms supplies and support will eventually grow into a larger commitment, because by itself supplying weapons likely won’t be enough to achieve anything except more Syrian deaths and a reduced chance of negotiating an end to the fighting. The U.S. already has more than enough commitments overseas, and the last thing it needs now is to be adding a new, difficult, long-term one in Syria.