This article asks the following questions. Which terrorism threats, challenges and responses did key players consider to have been decisively changed by 9/11? On close inspection now, nearly two decades after those attacks, how are we to assess such claims? What did 9/11 really change regarding terrorism and counter-terrorism? And what remained unaltered? The article’s central argument is this: some western states exaggerated the extent to which terrorist threats and challenges had been changed by 9/11 and, as a consequence, they did significantly alter some of their responses to terrorism; but at the heart of this ironic process was the tragic reality that, had there been a more serious-minded and historically sensitive recognition of how little had necessarily been changed by 9/11 in terms of terrorist threats and challenges, then the twenty-first-century experience of non-state terrorism would have been much less painful than has been the case in practice.