Insurance is a central, if until now ignored, instrument of war in the modern period. Ever since the eighteenth century, interaction between governments and insurers in Western countries has materialised in the form of war risk schemes that have contributed to the waging of war and the preservation of peace. The operation of those schemes has given rise to a curious, if not innocent, association between practices of statehood and practices of risk, which are theorised here under the label of insurantial sovereignty. The book draws on the British experience of using maritime insurance as an instrument of war during the Napoleonic Wars, the two World Wars, and the early twenty-first century. It asks, what happens, when, under conditions of war, the sovereign adopts insurantial imaginaries and practices into its rationalities of government? In doing so the book makes a novel contribution to the understanding of liberal security and liberal governance which is central to the theory of Political Science and International Relations, the understanding of international political sociology, and international political economy.