Yesterday evening I heard Trita Parsi talk about his book A Single Roll of the Dice: Obama's Diplomacy with Iran (Yale U.P.). Rather than summarizing the whole hour-long (including Q&A) talk, I will report what seemed to me to be the nub of his argument. This is thus a distillation and paraphrase (except where I use quotation marks):
Obama came into office in January '09 promising a new diplomatic approach to Iran. By the end of November '09 the approach had been abandoned, unwisely, for a "pressure track," i.e., escalating sanctions, which, unless changed, now puts the situation on a trajectory toward a war. Although there were various reasons for the change in policy (and the tale of course involves several key players other than Iran and the U.S.), Obama's main mistake was that he never tried to "create political space," in terms of U.S. domestic politics, for the diplomatic approach that the administration tried for less than a year. G.W. Bush gave 16 major speeches between Jan. '02 and March '03 making the (flawed) case for war with Iraq. By contrast Obama, although he spoke "passionately" as a candidate about the need for a diplomatic approach with Iran, as president publicly made the case for that approach only once (in a May '09 press conference with Netanyahu). The administration's unwillingness to spend political capital to defend its initial policy more or less guaranteed that the policy would have a short life and would not be given enough time to see whether it might have worked.
There was more in Parsi's talk (and obviously there is more in the book), but the above seemed to me to be the take-away message. He also has a recent short piece in The Nation along the same lines.