When a U.S. delegation traveled to Geneva in November last year for discussions with Iran over its nuclear program, it included Adam Szubin, director of the U.S. Treasury Department's Office of Foreign Assets Control, the U.S. government's sanctions enforcement unit.

His presence at this high-profile gathering, which led to an interim dealimplemented in January, was symbolic of the rising importance of OFAC. Sanctions have become a leading tool of U.S. foreign policy, with successive rounds of restrictions applied in recent...