With little fanfare, another taboo in U.S.-Iran relations has shattered. Jim Slattery, a former six-term Democratic Congressman from Kansas, late last year became the first former or current American legislator to visit the Islamic Republic of Iran. In Tehran to attend a conference on countering violent extremism, Slattery encountered a largely friendly reception from both officials and ordinary Iranians and came back equipped to present a more realistic and upbeat depiction of Iranians than is usually found on Capitol Hill. Speaking Monday at the Atlantic Council in Washington, Slattery conceded that there are elements in Iran that still oppose any...