Scientists have frequently imagined the borderless flow of knowledge as a norm and a natural consequence of the universality of scientific truth, but history has never been so simple.  Although natural philosophers created networks of correspondence that evolved into a self-defined “republic of letters” that supposedly transcended national borders, states simultaneously asserted prerogatives to control and regulate the flow of knowledge about the natural world long before the mid-twentieth century era of the national security state and the nuclear age. This presentation draws upon examples from the early modern period to the present to explore the political relationships that have conditioned international science and states’ attention to the movement of scientific information. According to this state-centered perspective, scientific exchange may be less about trust than about national interest and the geopolitical circumstances that either facilitate or obstruct knowledge flows across borders.