WASHINGTON — A great deal of diplomatic attention over the next few months will be focused on whether the temporary nuclear deal with Iran can be transformed into a full-blown accord. President Obama has staked the success of his foreign policy on this bold gamble. But discussion about the nuclear deal has diverted attention from an even riskier bet that Obama has placed: the idea that Iran can become a cooperative partner in regional security. Although they won’t say so publicly, Mr. Obama and Secretary of State John Kerry surely dream of a “Nixon to China” masterstroke. They are quietly...