Conspiracies play a significant role in world politics. States often engage in covert operations. They plot in secret, with and against each other. At the same time, conspiracies are often associated with irrational thinking and delusion. We address this puzzle and highlight the need to see conspiracies as more than just empirical phenomena. We argue that claims about conspiracies should be seen as narratives that are intrinsically linked to power relations and the production of foreign policy knowledge. We illustrate the links between conspiracies, legitimacy and power by examining multiple conspiracies associated with 9/11 and the War on Terror. Two trends are visible. On the one hand, US officials identified a range of conspiracies and presented them as legitimate and rational, even though some, such as the alleged covert development of weapons of mass destruction in Iraq, are now widely considered false. On the other hand, conspiracies circulating in the Arab-Muslim world were dismissed as irrational and pathological, even though some, like those concerned with the covert operation of US power in the Middle East, were based on credible concerns.