For Hezbollah, revenge seems to be a dish best served cold. After an explosive device concealed in a vehicle killed the Lebanese militant group's leader, Imad Mugniyah, on Feb. 12, 2008, reprisal attacks looked all but certain. Hezbollah, after all, could hardly let such a high-level assassination go unanswered. Then on June 8 of this year, nearly a decade after Mugniyah's death, the U.S. Department of Justice announced that it had arrested two Hezbollah operatives in New York and Michigan. One of the suspects, Ali Kourani, appears to have played a part in planning the long-awaited retaliatory strike. The criminal complaint against him accuses Kourani of conducting surveillance against various targets, including individuals connected to the Israel Defense Forces, in preparation for an attack avenging Mugniyah. Though the attack never materialized, the information Kourani passed back to Lebanon in the course of his surveillance operations could provide the basis for...