The study presents a force theory for incompressible flow about several solid bodies, which enables us to examine the force contribution to each body from individual fluid elements. By employing auxiliary potential functions, we decompose hydrodynamic forces in terms of the unsteadiness of the incoming stream, vorticity within the flow, and surface vorticity on the solid bodies. The usefulness of this force decomposition is illustrated by examining separated flow about several circular cylinders. Guidelines were obtained for finding an optimal arrangement to achieve significantly small drag exerted on the cylinders.