The Bush administration claims that the law of war and Supreme Court precedent support the president's authority to detain U.S. citizens, incommunicado and without filing a criminal charge, as “enemy combatants.” The administration views this power as inherent in the president's commander-in-chief authority, and that congressional authorization, while unnecessary, is implied in statute. This article surveys the history of presidential efforts to deal with threats to the national security through preventive detention measures, and concludes that congressional authorization has always been necessary to validate such measures within the United States.