10 USC 3062 - Policy; composition; organized peace establishment

(a) It is the intent of Congress to provide an Army that is capable, in conjunction with the other armed forces, of
(1) preserving the peace and security, and providing for the defense, of the United States, the Commonwealths and possessions, and any areas occupied by the United States;
(2) supporting the national policies;
(3) implementing the national objectives; and

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(4) overcoming any nations responsible for aggressive acts that imperil the peace and security of the United States.
(b) In general, the Army, within the Department of the Army, includes land combat and service forces and such aviation and water transport as may be organic therein. It shall be organized, trained, and equipped primarily for prompt and sustained combat incident to operations on land. It is responsible for the preparation of land forces necessary for the effective prosecution of war except as otherwise assigned and, in accordance with integrated joint mobilization plans, for the expansion of the peacetime components of the Army to meet the needs of war.
(c) The Army consists of
(1) the Regular Army, the Army National Guard of the United States, the Army National Guard while in the service of the United States and the Army Reserve; and
(2) all persons appointed or enlisted in, or conscripted into, the Army without component.
(d) The organized peace establishment of the Army consists of all
(1) military organizations of the Army with their installations and supporting and auxiliary elements, including combat, training, administrative, and logistic elements; and
(2) members of the Army, including those not assigned to units;

necessary to form the basis for a complete and immediate mobilization for the national defense in the event of a national emergency.