Marc on the secret letter Obama sent to Medvedev:
The connective tissue between Russia and Iran is tough; Russia's support for the Iranian regime is based on leverage and mutual interest; it cannot be snipped until the benefits of the longstanding alliance (a market for Russian goods, non-interference by Iran in Muslim parts of the old Soviet empire, a counterbalance to U.S. hegemony) outweight the costs.
What are the costs? A nuclear Iran may become less dependent on Russia. Others are harder to figure out. And so the truth is that Russia has a stronger hand here; Russia realizes this and Medvedev seems to be in an expansive mood, willing to concede that because his country's support for Iran is based on practical (material) concerns rather than ideology, Russia can use its influence productively. In this way, the "strategic stability" between the U.S. and Russia depends quite a bit on the markers the world's major economic powers lay down this week.