After 9/11, American officials authorized numerous contentious counterterrorism practices including torture, extraordinary rendition, indefinite detention, trial by military commission, targeted killing, and mass surveillance. While these policies sparked global outrage, the Bush administration defended them as legally legitimate. Government lawyers produced memoranda deeming enhanced interrogation techniques, denial of habeas corpus, drone strikes, and warrantless wiretapping lawful. Although it rejected torture, the Obama administration made similar claims and declined to prosecute abuses. This book seeks to understand how and why Americans repeatedly legally justified seemingly illegal security policies and what this tells us about the capacity of law to constrain state violence. It argues that legal cultures shape how political actors interpret, enact, and evade legal norms. In the global war on terror, a culture of legal rationalization encouraged authorities to seek legal cover—to construct the plausible legality of human rights violations—in order to ensure impunity for wrongdoing. In this context, law served as a permissive constraint, enabling abuses while imposing some limits on what could be plausibly legalized. Cultures of legal rationalization stand in contrast with other cultures prevalent in American history, including cultures of exception, which rely on logics of necessity and racial exclusion, and cultures of secrecy, which employ plausible deniability. Looking forward, legal norms remain vulnerable to manipulation and evasion. Despite the efforts of human rights advocates to encourage deeper compliance, the normalization of post-9/11 policy has created space for the Trump administration to promote a renewed culture of exception and launch bolder attacks on the rule of law.