What causes imperialism? Classic explanations of imperialism theorized causes within the imperial metropole, such as nationalist culture or the imperatives of capital accumulation. More recent theories emphasize global pressures. To assess these longstanding but rarely adjudicated theories, this paper employs original time-series data and qualitative evidence on British imperialism from the eighteenth to the mid-twentieth century. The analysis shows that British imperialism was not only a domestically produced process but also a global one. British imperialism was a strategy for meeting the state’s imperatives of geopolitical security (containment) and economic growth (capital) in the face of external threats. Those threats were most likely to be perceived when the global system was at its most economically competitive. These findings affirm a “structural competition” approach that builds upon existing global-oriented theories but specifies the structural conditions and causal processes driving imperialism.