What determines the effectiveness of military buildups? We introduce the concept of the military multiplier: the percentage increase in military equipment an additional dollar buys. It varies with the costs of allocating resources to military production, depending, among other things, on the industrial structure and capital reallocation frictions. We show that the response of military-goods prices to military buildups is a sufficient statistic for the military multiplier and that it has declined over time in the US. Using a calibrated multi-sector business cycle model, we show this decline stems from the economy’s structural shift toward the service sector.