In 2015, the United States military struck a hospital in Afghanistan run by Médecins Sans Frontières, killing forty-two staff and patients. Testifying afterwards before a Senate Committee, General John F. Campbell explained that “[t]he hospital was mistakenly struck.†In 2019, while providing air support to partner forces under attack by ISIS, the U.S. military killed dozens of women and children. Central Command concluded that any civilian deaths “were accidental.†In August 2021, during a rushed withdrawal from Afghanistan, the U.S. military executed a drone strike in Kabul that killed ten civilians, including an aid worker for a U.S. charity and seven children in his family. The Pentagon later admitted it was a “tragic mistake.†In these cases and others like them, no one set out to kill the civilians who died. Such events are usually chalked up as sad but inevitable consequences of war— as regrettable “mistakes.†This Article examines the law on “mistakes†in war. It shows that, while those responsible for “mistakes†are often not held accountable, there is significant evidence that certain “mistakes†are criminally culpable. It also shows that the law holds not just individuals, but also States, responsible for preventing certain “mistakes.†To see how the law works, or fails to work, in practice, the Article examines the U.S. military’s own assessments of civilian casualties. The analysis focuses on the United States, both because of its global military operations and because of the power of its example to shape global practices. It demonstrates that “mistakes†in the U.S. counterterrorism campaign have been far more common than generally acknowledged. Moreover, it argues that the repeated errors that these reports reveal— not just one mistake, but an unmistakable pattern of mistakes—are the predictable result of a system that, during the period examined, did too little to learn from its mistakes. It concludes by advocating reforms to address individual and systemic ...