As the nation’s spy agencies assess the fallout from disclosures about their surveillance programs, some government analysts and senior officials have made a startling finding: the impact of a leaked terrorist plot by Al Qaeda in August has caused more immediate damage to American counterterrorism efforts than the thousands of classified documents disclosed by Edward Snowden, the former National Security Agency contractor. Since news reports in early August revealed that the United States intercepted messages between Ayman al-Zawahri, who succeeded Osama bin Laden as the head of Al Qaeda, and Nasser al-Wuhayshi, the head of the Yemen-based Al Qaeda in...