This is an article on Swedish coastal defence over four centuries. It seeks to understand the changes in Swedish naval policy over time by exploring how the understanding of the nature of war visible in defence planning varies over time, due to both changing norms and changes in harder factors, such as geography, resources and adversaries’ capabilities. Its primary aim is to account for the development of Sweden’s naval capacities from the mid-seventeenth century onwards. For this, the article draws on sea power concepts. Its secondary aim is to explore the development of war as an institution of international society by studying only one – albeit historically very central – aspect of one state’s warring capacity over time, namely its navy.