The backbone of the air transportation system for the US military is the En Route System (ERS), a collection of personnel and equipment at bases throughout the world. The location of these bases in the ERS bears the marks of the Cold War legacy of the latter half of the twentieth century. While many changes to the ERS have taken place in the last 20 years, it still has a predominantly East-West orientation in the Northern Hemisphere with little capability in many parts of the world. This research provides the decision makers (DMs) with a clear methodology for evaluating the best locations for new en route bases in the ERS. It looks at the capability of each en route airfield alternative using 27 different measures. It then examines the National Security Import of the destination regions an en route airfield could service, along with the probability of events in those regions requiring military action, including humanitarian and other missions.