The U.S. Military’s Force Structure: A Primer, July 2016: “The U.S. military’s capabilities and budget are determined primarily by the number and type of combat units, as well as the personnel and equipment they contain. This report describes each type of major combat unit’s structure, function, strengths and limitations, past usage, size, and cost, accounting for supporting units and overhead. That information can be used to compare elements of the force, understand the composition of the Defense Department’s budget, and calculate the costs or savings that would result from changing the size or composition of the armed forces…In fiscal year 2015, the Department of Defense (DoD) spent a total of roughly $390 billion on operation and support (O&S) of military units. The O&S budget covers the costs associated with the day-to-day running of units. Those costs include pay and benefits for military personnel, compensation for most civilian employees, health care costs for military and civilian personnel, and the daily expenses of operating a unit, such as equipment maintenance, training, support contractors, and so on. The O&S budget makes up about two-thirds of DoD’s total “base” budget, which is the defense budget excluding additional funds provided specifically for wartime operations. (The rest of DoD’s base budget is spent on acquiring weapon systems and constructing buildings and other infrastructure.)”