When British Paratroopers shot dead 13 people at a civil rights march in Derry on January 30, 1972 it dealt a hammer blow to British government claims of neutrality and moral authority in dealing with the escalating violence in Northern Ireland. Existing historical accounts of Bloody Sunday treat the killings as the outcome of a more-or-less unified military anxiety at increasing disorder in Derry, combined with unexpected events on the day, presenting the killings as the outcome of essentially responsive actions by the British military. In so doing they lend support to the 'cock-up' theory that represents the killings as the outcome of a series of errors of interpretation and communication. This article provides an alternative interpretation of the political and military decision-making process, challenging key elements in the analysis in the existing literature. By contrast with existing accounts, it argues that the Bloody Sunday operation was a calculated plan devised at a very high level to stage a massive and unprecedented confrontation that would disrupt and shatter an established policy of security force restraint in the city of Derry. It argues further that the operation that day emerged from an intense internal struggle to shape security policy that reflected deep divisions within the security forces, analysing the statements and evidence of key participants much more critically than existing accounts do. It argues that high-level decision-making is central to the explanation of the outcome that day and that the operation raises serious questions about the relationship between political decision-making and the operational decision-making of the army in Northern Ireland.