In case we forgot why, Dennis Ross reminds all of us why we should ignore what he writes about Iran policy:

First, Iran’s President Rouhani, who continues to send signals that he wants to make a deal on the nuclear program, will inevitably be weakened once it becomes clear that the U.S. cannot use force against Syria [bold mine-DL].

This is entirely backwards. Not attacking an Iranian ally creates an opening for Rouhani to make conciliatory moves, however small, that make a deal more likely. Attacking Syria would confirm Iranian leaders’ suspicions that the U.S. is unremittingly hostile to Iran’s interests and that nuclear weapons are the only deterrent against U.S. attack. Why would Iran would become more willing to make a deal on the nuclear issue when their president’s conciliatory gestures have been answered by a military strike on an allied government? You can be sure that American hard-liners would use evidence of hostile Iranian actions as proof that negotiation is impossible and the other government is not to be trusted. If Ross misunderstands this most elementary political dynamic, why should anyone trust his recommendations for Syria or Iran policy?

If attacking Syria makes Iran more interested in acquiring a nuclear weapon, and if it contributes to rising tensions with the U.S. and Israel as a result, it will have helped make the disastrous outcome of war with Iran that much more likely. That would be bad for all parties, but it would be worst for Iranian advocates for political reform and accommodation with the U.S. If the U.S. wishes to help Iranian hard-liners marginalize and discredit Rouhani, then it should attack Syria.