ribit, the problem is Iran is increasingly beligerent and emboldened by trading partners Russia and China at their back. They likely feel safe as long as they are not the first to attack. Russia's been making a lot of noise that if Iran is attacked, it will be a threat to their national security.
My take is Russia's a small country now and if they attack US in defense of Iran, they could be left with nothing. Will Russia back down or start a real world war? I think they'd back down. It becomes a moot point if Iran strikes first.