Imagine three Pittsburghs, simultaneously — a multi-pronged murderous attack at Jewish institutions across the country. The Jewish community’s top security arm did just that last week at an exercise joining community leaders from across the country with senior U.S. law enforcement officials. Six months after the massacre of 11 Jewish worshippers at the Tree of Life synagogue complex in Pittsburgh, several dozen officials met in a conference room overlooking the National Mall here and reviewed the lessons of that awful day as a means of dealing with future attacks. The most surprising and poignant recommendations came from survivors of the...