Political power in the international context can be characterized as a fluid-like substance that circulates through a network of nation states. States can possess it as a stock quantity, reflected by their material capacity or national wealth; and they can transfer it as a flow quantity, through constructive or destructive action. Constructive activities like trade increase a state's power, while destructive ones like violent conflict reduce it. In this paper, we quantify these assertions to a first approximation using economic and military data, parameterizing a mathematical model that can forecast the evolution of power in the international system.