A great power war remained a possibility in Europe in the early twentieth century and preparing for it remained the state’s most important responsibility before 1914. This chapter examines how states developed plans to fight under conditions of economic interdependence. Preparations sought to mitigate the risks to economic stability and exploit the enemy’s vulnerabilities. The lack of prior experience of a major European war in an interdependent global economy rendered planning extremely difficult. Some challenges went unforeseen; others required trade-offs that had to be balanced against the peace-time economy. These preparations imposed another interdependence dilemma on states – war preparations were costly, detracting from the economic basis of a state’s future power. Undertaking the preparations, inadequate and superficial as they proved, only served to underline the catastrophic consequences of war and spread knowledge about the high risks across bureaucracies and civil society. Planning for war, though dominated by military and naval leaders, included a wide range of civilian officials and experts from civil society.