At this highly unpredictable time in world politics, commentators are increasingly turning to the past to make sense of the present (I’m just as guilty as many far more eminent figures). As part of this trend the so-called ‘Thucydides trap’ has recently come to prominence, thanks to a project run by Harvard University’s Belfer Center and a forthcoming book by its director, Graham Allison. Allison is interested in the Greek historian Thucydides’ statement (at 1.23.6 of the History of the Peloponnesian War) that the Peloponnesian War between Athens and Sparta (431-404 BC) was inevitable, because of the fear felt by the Spartans, an established power, of the rising Athenians. For him, this handful of words is a key to more recent historical confrontations, and the project website contains brief analyses of 16 other rivalries between ruling and rising powers between 1500 and today, making the point that 12 of these ended in war; readers can suggest other examples to be considered as part of the second phase of the project. Allison’s main thesis is that what happened with Sparta and Athens is a cautionary tale for today’s US and the threat allegedly posed to it by an ascendant China: it is in fact ‘the best lens for understanding the most critical foreign policy issue of our time’.