Why do surprise attacks-whether from terrorists or from conventional enemies-succeed even when some advance warning frequently seems clear in hindsight? Ever since Pearl Harbor the conventional wisdom has been that surprise attacks succeed because intelligence and national security officials lack the imagination or capacity to "connect the dots" from the available information or lack "game-changing" strategic intelligence. But this work argues that the conventional wisdom is wrong: by comparing cases where intelligence failed to anticipate and stop a surprise attack with cases where intelligence did prevent the attack, Dahl find that the key to success is not more imagination. Rather the acquisition of specific, tactical-level intelligence, combined with the presence of decisionmakers who are receptive to the warnings they are given makes the difference. Strategic intelligence is often what decisionmakres say they want, but Dahl finds that in practice, strategic intelligence is generally non-specific and thus doesn't foster a sense of urgency to act. This book offers a theory of preventive action and advances the literatures on intelligence and surprise attack.