Congress is given express authority "To raise and support armies" with the provision that "no appropriation of money to that use shall be for a longer term than two years " and " To provide and maintain a navy " (Const. Art. I, § 8, ¶¶ 12, 13). The reason for the provision that appropriations for the support of the army must be for a limited period only has been already suggested. (See above, § 69.) The armies may consist of the state militia called into the federal service; but Congress is not limited to this method of raising armies, and may provide for the enlistment of regular federal troops, or if deemed necessary, for compelling service by conscription, and the calling out of the state militia as such has been found to be inexpedient save for temporary purposes. The United States maintains a regular army, and in time of war provides for additional forces by the enlistment of volunteer regiments which may be enlisted and recruited in the various states as Congress shall provide.

Regiments of state militia may be enlisted as volunteer regiments. There is no practical distinction between the militia of the various states called into the service of the United States, and the other military forces of the United States, save as to their original organization and the appointment of subordinate officers. As the states are prohibited from having ships of war, the federal navy is composed entirely of vessels belonging to, or for the time being employed in the service of the United States under its authority. The powers of the president as to the army and navy will be discussed in a subsequent chapter. (See below, § 130.)

The power to grant letters of marque and reprisal (Const. Art. I, § 8, ¶ 11) involves the granting to private vessels, sailing under the flag of the United States, the authority to capture enemies' vessels and other property upon the high seas. Indeed, reprisals may be authorized without the existence of a state of war, and for the purpose of securing from a foreign country or its citizens, redress for grievances; but the making of reprisals is an initial step towards the commencement of hostilities, and may therefore be properly regarded as incident to the war-making power. Private vessels to which letters of marque and reprisal have been granted, usually denominated privateers, act under the authority of the government granting such letters, but are not, properly speaking, a part of its navy.