Answer Me This

I know it's not a quid pro quo, blah, blah, blah, but the deal is: Obama offers an end to missile defense if the Russians ensure the Iranians don't get the bomb. Okay. But what if Russia says "No." Then is Obama prepared to go ahead with missile defense? And then how exactly is Obama proposing anything different from Bush? And if he's still not willing to move ahead on missile defense then why do the Russians agree to help with the Iranians?