At the same time, the evolving political crisis in Iran has undercut any prospects for Obama's policy of "engagement" with Tehran. Although the regime has forcibly squelched mass demonstrations, internal divisions within the elite are still playing themselves out.

In these circumstances, it's hard to discern whom one would talk with, and to what purpose. The hard-line paramilitary Revolutionary Guards, loyal to Ayatollah Ali Khamenei, have clearly gained in power and seem to have little incentive to compromise at home or abroad.

A senior U.S. defense official recently told me that, in the spring of 2008, the commander of the Quds force, a secretive unit of the Revolutionary Guards, sent a message to Gen. David Petraeus. The message said Petraeus should know that "I, Brigadier General Qassim Suleimani, control the foreign policy of my country with respect to Iraq, Afghanistan, Lebanon and Gaza." In other words, forget the Iranian foreign ministry, Ahmadinejad or even the supreme leader — the Quds force controls much of Iranian foreign policy.

And no one is entirely certain, following recent events, who controls Iran's nuclear portfolio.

With Obama's engagement policy on ice and tougher sanctions hard to envision, Iran analysts got a jolt on Sunday, when Vice President Joe Biden seemed to say on ABC's "This Week" that America would give Israel the green light to attack Iran's nuclear installations.

Tuesday, in Moscow, Obama walked Biden's remarks back. The United States is "absolutely not" giving Israel a green light to attack Iran, he told CNN. Glad to hear that. I think Biden got it right, however, when he said, "We're not rushing to sit down (with Tehran)."

This is a time for Obama to breathe deeply, watch developments in Tehran, and keep pressing Russia to act in its own best interest on Iran. And to start thinking, if all else fails, about a Plan B.

Trudy Rubin is a columnist and editorial-board member for the Philadelphia Inquirer. Readers may write to her at: Philadelphia Inquirer, P.O. Box 8263, Philadelphia, PA 19101, or by e-mail at trubin@phillynews.com.