Article One of the United States Constitution
Under the Compact Clause, states may not, without the consent of Congress, keep troops or armies during times of peace. They may not enter into alliances nor compacts with foreign states, nor engage in war unless invaded. States may, however, organize and arm a militia according to the discipline prescribed by Congress. (Article I, Section 8, enumerated powers of Congress.) The National Guard, whose members are also members of the militia of the United States as defined by , fulfill this function, as do persons serving in State Militias with federal oversight under .