The National Guard Act (1903):
"[T]he militia shall consist of every able-bodied male citizen of the respective States, territories, and the District of Columbia, and every able-bodied male of foreign birth who has declared his intention to become a citizen, who is more than eighteen and less than forty-five years of age, and shall be divided into two classes -- the organized militia, to be know as the National Guard of the State, Territory, or District of Columbia, or by such other designations as may be given them by the laws of the respective States or Territories, and the remainder to be know as the Reserve Militia." [from "An Act To promote the efficiency of the militia, and for other purposes", January 21, 1903]
10 USC Sec. 311:
"(a) The militia of the United States consists of all able-bodied males at least 17 year of age and, except as provided in section 313 of title 32 [32 USC sec. 313], under 45 years of age who are, or who have made a declaration of intention to become, citizens of the United States and of female citizens of the United States who are commissioned officers of the National Guard.
(b) The classes of the militia are --
the organized militia, which consists of the National Guard and the Naval Militia; and
the unorganized militia, which consists of the members of the militia who are not members of the National Guard or the Naval Militia."
US Code, Title 10 section 312:
(a) The following persons are exempt from militia duty:
The Vice President.
The judicial and executive officers of the United States, the several States and Territories, and Puerto Rico.
Members of the armed forces, except members who are not on active duty.
Customhouse clerks.
Persons employed by the United States in the transmission of mail.
Workmen employed in armories, arsenals, and naval shipyards of the United States.
Pilots on navigable waters.
Mariners in the sea service of a citizen of, or a merchant in, the United States.
(b) A person who claims exemption because of religious belief is exempt from militia duty in a combatant capacity, if the conscientious holding of that belief is established under such regulations as the President may prescribe. However, such a person is not exempt from militia duty that the President determines to be noncombatant.
Supreme Court, U.S. vs. Miller (1939):
"The signification attributed to the term Militia appear from the debates in the Convention, the history and legislation of Colonies and States, and the writings of approved commentators. These show plainly enough that the Militia comprised all males physically capable of acting in
concert for the common defense..."
"[s]ince 1933 all persons who have enlisted in a state National Guard unit have simultaneously enlisted in the National Guard of the United States. In the latter capacity they became a part of the Enlisted Reserve Corps of the Army, but unless and until ordered to active duty in the Army they retained their status as members of a separate state Guard unit... Notwithstanding the brief periods of federal service, the members of the state Guard unit continue to satisfy this description of the militia. In a sense, all of them now must keep three hats in their closets--a civilian hat, a state militia hat, and an army hat--only one of which is worn at any particular time.
When the state militia hat is being worn, the 'drilling and other exercises' referred to by the Illinois Supreme Court are performed pursuant to 'the Authority of training the Militia according to the discipline prescribed by Congress,' {U.S. Constitution, Article I, section 8, clause 16} but when that hat is replaced by the federal hat, the Militia Clause is no longer applicable."
Perpich, Governor of Minnesota, et.al. v. Department of Defense et.al.
(No. 89-542) U.S., 110 S.Ct. 2418, 110 L.Ed.2d 312 (1990).