10 U.S.C. § 12103

(a) Except as otherwise prescribed by law, enlistments as Reserves are for terms prescribed by the Secretary concerned. However, an enlistment that is in effect at the beginning of a war or of a national emergency declared by Congress, or entered into during such a war or emergency, and that would otherwise expire, continues in effect until the expiration of six months after the end of that war or emergency, whichever is later, unless sooner terminated by the Secretary concerned. (b) Under regulations to be prescribed by the Secretary of Defense, and by the Secretary of Homeland Security with respect to the Coast Guard when it is not operating as a service in the Navy, a person who is qualified for enlistment for active duty in an armed force, and who is not under orders to report for induction into an armed force under the Military Selective Service Act ( 50 U.S.C. 3801 et seq.), may be enlisted as a Reserve for service in the Army Reserve, Navy Reserve, Air Force Reserve, Marine Corps Reserve, or Coast Guard Reserve, for a term of not less than six years nor more than eight years. Each person enlisted under this subsection shall serve— (1) on active duty for a period of not less than two years; and (2) the rest of his period of enlistment as a member of the Ready Reserve. (c) In time of war or of national emergency declared by Congress the term of service of an enlisted member transferred to a reserve component according to law, that would otherwise expire, continues until the expiration of six months after the end of that war or emergency, whichever is later, unless sooner terminated by the Secretary concerned. (d) Under regulations to be prescribed by the Secretary of Defense, or the Secretary of Homeland Security with respect to the Coast Guard when it is not operating as a service in the Navy, a non-prior-service person who is qualified for induction for active duty in an armed force and who is not under orders to report for induction into an armed force under the Military Selective Service Act ( 50 U.S.C. 3801 et seq.), except as provided in clauses (ii) and (iii) of section 6(c)(2)(A) of such Act ( 50 U.S.C. 3806(c)(2)(A) ), may be enlisted in the Army National Guard or the Air National Guard, or as a Reserve for service in the Army Reserve, Navy Reserve, Air Force Reserve, Marine Corps Reserve, or Coast Guard Reserve, for a term of not less than six years nor more than eight years. Each person enlisted under this subsection shall perform an initial period of active duty for training of not less than twelve weeks to commence insofar as practicable within one year after the date of that enlistment.