Engaging Iran Worked

Albeit indirectly:

Perhaps the biggest achievement of negotiations to date has been their facilitation of the imposition of tough international sanctions against the Iranian government and its nuclear program. As a recent IAEA report revealed, these sanctions have been instrumental in slowing the progress of the Iranian program. This view is now shared by former Israeli defense officials such as Gabi Ashkenazi, who in a speech at the Brookings Institution stated that sanctions were the best course of action against Iran. Meanwhile, in a recent visit to the IAEA, Israel's Deputy Prime Minister Dan Meridor acknowledged that sanctions against Iran could work. Another Israeli security official, speaking on background in an interview with the authors, said that, while Israelis initially "were skeptical about a possible positive outcome of the negotiations" in respect to the nuclear issue, "we recognize that they contributed to building international consensus." The fact that such statements are being made by officials of a country skeptical of sanctions speaks volumes.

And it was Obama's steadiness that brought this about. Contrast that with the clear indications from both Perry and Romney that they would encourage Israel to launch a military attack.