A militia is a military force raised from the civilian population to supplement the regular army in the event of an emergency. Militias evolved over time from a state institution into the National Guard system. Individuals in the state National Guards also are in the National Guard of the United States under a dual enlistment theory. Under the Constitution, National Guard members can be called up to military service in the case of a national emergency.
[The Congress shall have Power . . . ] To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions; . . .
United States Library of Congress, The Constitution of the United States of America: Analysis and Interpretation
The power of Congress over the militia "being unlimited, except in the two particulars of officering and training them . . . it may be exercised to any extent that may be deemed necessary by Congress. . . . The power of the state government to legislate on the same subjects, having existed prior to the formation of the Constitution, and not having been prohibited by that instrument, it remains with the States, subordinate nevertheless to the paramount law of the General Government. . . ."1 Under the National Defense Act of 1916,2 the militia, which had been an almost purely state institution, was brought under the control of the National Government. The term "militia of the United States" was defined to comprehend "all able-bodied male citizens of the United States and all other able-bodied males who have . . . declared their intention to become citizens of the United States," between the ages of eighteen and forty-five. The act reorganized the National Guard, determined its size in proportion to the population of the several States, required that all enlistments be for "three years in service and three years in reserve," limited the appointment of officers to those who "shall have successfully passed such tests as to . . . physical, moral and professional fitness as the President shall prescribe," and authorized the President in certain emergencies to "draft into the military service of the United States to serve therein for the period of the war unless sooner discharged, any or all members of the National Guard and National Guard Reserve," who thereupon should "stand discharged from the militia."3
The militia clauses do not constrain Congress in raising and supporting a national army. The Court has approved the system of "dual enlistment," under which persons enlisted in state militia (National Guard) units simultaneously enlist in the National Guard of the United States, and, when called to active duty in the federal service, are relieved of their status in the state militia. Consequently, the restrictions in the first militia clause have no application to the federalized National Guard; there is no constitutional requirement that state governors hold veto power over federal duty training conducted outside the United States or that a national emergency be declared before such training may take place.4
The states, as well as Congress, may prescribe penalties for failure to obey the President's call of the militia. They also have a concurrent power to aid the National Government by calls under their own authority, and in emergencies may use the militia to put down an armed insurrection.5 The Federal Government may call out the militia in case of civil war; its authority to suppress rebellion is found in the power to suppress insurrection and to carry on war.6 The act of February 28, 1795,7 which delegated to the President the power to call out the militia, was held constitutional.8 A militiaman who refused to obey such a call was not "employed in the service of the United States so as to be subject to the article of war," but was liable to be tried for disobedience of the act of 1795.9