https://constitution.congress.gov/browse/essay/artI-S8-C15-1/ALDE_00001077/Article I, Section 8, Clause 15:
[The Congress shall have Power . . . ] To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions; . . .
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 armed insurrection.1 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.2 The act of February 28, 1795,3 which delegated to the President the power to call out the militia, was held constitutional.4 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 1795.5
Article I, Section 8, Clause 16:
[The Congress shall have Power . . . ] To provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress; . . .
The Supreme Court has characterized Congress’s power over the militia as being unlimited, except in the two particulars of officering and training them under the Militia Clauses,1 such that the power may be exercised to any extent that may be deemed necessary by Congress.2 At the same time, the Court acknowledged [t]he power of the state government to legislate on the same subjects, having existed prior to the formation of the Constitution remained with the states.3 However, this power, the Court continued, is nevertheless subordinate to the paramount law of the General Government.4
Under the National Defense Act of 1916,5 the militia, which had been an almost purely state institution, was brought under the control of the federal government. The act divided militia of the United States—defined to include 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—into several classes of organized militias, including the National Guard. Among its measures, 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, and 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.6 The act also 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.7
The Militia Clauses do not constrain Congress in raising and supporting a national army. The Supreme 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.8 Consequently, the restrictions in the first militia clause that limit the militia to be called forth for three specified purposes do not apply to the federalized National Guard.9 Nor is there a constitutional requirement that state governors hold a veto power over federal duty training conducted outside the United States or that a national emergency be declared before such training may take place.10