Economic modernisation is widely seen as a path to democracy, but the technological progress that drives modernisation also provides rulers with new means of repression. We collect data on the international diffusion of 29 repressive military technologies, and demonstrate that such technologies spread faster from Western Europe and the United States than economic development. Moreover, in a panel of all independent countries in the period 1820–2010, we show that the rapid diffusion of repressive technologies has impeded democratisation around the world, by allowing autocratic rulers to suppress popular resistance against their regimes.