Each nation's constitution determines how the federal and state governments are divided up in terms of authority.
Federal systems, such as the United States, are characterized by limited powers granted to state governments by the constitution. Although state governments maintain National Guard forces, the federal government retains ultimate military authority.
Local governments under unitary systems, such as the United Kingdom, are subordinate to the central government and lack autonomous military authority. One important aspect that is dictated by the constitutions of each nation is the level of autonomy that is allowed to state or regional administrations, including their military capabilities.