“Deterrence, in the Cold War, was a relatively straightforward concept to understand,” said one of the speakers at the recent CHACR workshop that considered deterrence, and that this edition of Ares & Athena summarises. He was, of course, right – but only because he inserted that key word ‘relatively’ into the statement. Because deterrence, in the Cold War, was a very difficult and complex subject: hard to articulate and hard to understand. Indeed, the most eminent of academics, like Sir Lawrence Freedman for example, built their careers upon a bedrock of Cold War deterrence theory. The thrust of what the speaker had to say, therefore, was that it is now much harder to understand a concept that was already hard to understand. So it is not a subject that can be ‘solved’ by the tidy-minded and set aside in a neat piece of defence doctrine. It is a subject that should become, once again, central to the discussions, debates and reflections of defence professionals.