As our centennial series continues, and on the anniversary of the 9/11 attacks, Richard English, professor of politics at Queen's University Belfast where he directs the Senator George J. Mitchell Institute, the editor of the Cambridge History of Terrorism (Cambridge University Press, 2021) and the author of Does Terrorism Work?: A History (Oxford University Press, 2016) and Does Counter-Terrorism Work? (OUP, 2024), reviews 100 years of the use of terrorism to achieve political aims.