This article looks again at the ‘Peterloo massacre’ of 16 August 1819 in Manchester, and offers new evidence from the Home Office Disturbances Papers about how it came about and who was responsible. In Section I the revisionist case that Peterloo was a conflict rather than a massacre is examined and found wanting. Section II argues that in the months before Peterloo, the Home Office had consistently urged the Lancashire magistrates to combat radical agitation ‘either by the law or the sword’; its well-known advice for restraint on 16 August was a piece of qualified back-pedalling which came too late. It also argues that the authorities misinterpreted the peaceful mass platform agitation of 1819 in the light of their experience of the attempted insurrections of 1817. Section III shows that the prosecution at the Peterloo trial privately conceded that its own version of events had been discredited. Section IV suggests that both conservative and progressive versions of what happened at Peterloo are constricted by whiggish assumptions, proposes some questions for further study, and offers a brief summary explanation of the Peterloo massacre.