As Joel Braunold points out in his Open Zion article, intervening in Syria is both a security imperative and a risk. I agree with everything he says, but I think he missed the most important reason to intervene in Syria. Israel is watching this very carefully to see if Obama's red line in Syria means anything. If we fail to launch some sort of attack (preferably as part of a multinational force), then Israel will conclude that we have a backbone made of silly putty. Israel will conclude that it can not count on us to keep our word when we say "THIS IS A RED LINE THAT MUST NOT BE CROSSED!" And this creates a dangerous situation.
If Israel sees we are afraid to enforce our red line here, how will it trust us when we say "Hold off on attacking Iran. There is still a bit more time before Iran gets a nuke. Don't attack prematurely and we promise we will attack if they get too close to assembling a weapon."
Short answer is, Israel won't be able to trust us. And I am convinced it is our promise to step in at the last minute that has prevented Israel from launching an attack against Iran.
So we must attack Syria in order to prevent Israel from launching an attack against Iran--an attack that is extremely risky and is only likely to delay Iran's getting the bomb by a few years under the best case scenario.
The question is, HOW can we attack Syria without triggering retaliation from Iran or causing a total collapse of nuclear negotiations?
We should lift at least SOME sanctions on Iran as we get ready to attack Syria. They don't have to be major sanctions, but we should at least throw them some type of bone. Lifting sanctions is difficult. Most sanctions will require an act of Corngress and/or international cooperation to lift. But we could at least state that we won't be as rigorous at enforcing some of the sanctions as we have been.
Then we should tell the Iranians that the sanctions will be restored if they retaliate against us for attacking Assad or if they refuse to continue nuclear talks. This needs to be followed with a promise to lift more sanctions if serious progress is being made by nuclear talks.
If we show the Iranians a good faith gesture they may be more inclined to let a (limited) attack against Assad go without much protest. IF this is really Iranian President Rouhani's Twitter account then he MAY be signaling that Iran will tolerate at least a limited attack against Assad. At least, that's what Jasmin Ramsey at LobeLog thinks.
Of course, he didn't explicitly spell out who he thinks is responsible for the gas attacks. If he thinks the rebels are responsible then this might not be an open door for intervention. But lifting some sanctions would give him an opportunity to clarify his statement. We can always re-impose them if he was hinting that we should attack the rebels. And if Rouhani isn't blowing smoke about being more flexible in nuke talks, then modest sanctions relief might empower him to stand up to those who want the talks to go nowhere.
Update: It looks like Rouhani is now trying to put the breaks on U.S. intervention in Syria