America is in a long war against a resilient enemy capable of striking the homeland, but U.S. intelligence capabilities are falling short of meeting the threat. The San Bernardino attackers were not flagged, despite their repeated visits to jihadist websites, alarming posts on social media, and suspicious financial transactions. The Boston Marathon bombers evaded timely detection, as did the would-be shooters in Garland, Texas, who had exchanged dozens of messages with a known terrorist overseas.

Paris and San Bernardino exemplify...