Interest in energy transitions has accelerated in recent years due to rising concerns about global warming and resource scarcity, but the drivers of these phenomena are not well understood. To date, scholars have primarily focused on commercial and technological factors, highlighting that oil was ‘better’ than coal – more powerful, cheaper, cleaner, and more practical to use – and that the internal combustion engine made it more advantageous to use in transportation. Yet oil was also a strategic commodity that powerful states sought to acquire for military reasons. This article contends that geopolitics, military decision-making, and energy security hastened the transition from oil to coal prior to the First World War. It argues that Britain, Germany, and the United States sought to transition their naval fleets from coal to oil to gain a military advantage at sea, which created, for the first time, the problem of oil-supply security. Through government-led initiatives to address oil-supply security, vast new supplies of oil came online and prices fell, the ideal environment for oil to eclipse coal as the dominant source in the global energy system.