The Constitution enumerates three purposes for aid in the effectuation of which the United States militia forces may be mandatorily called upon by the General Government. These are (1) to execute the laws of the Union, (2) to suppress insurrections, (3) to repel invasions.

The suppression of insurrections has been held to include the waging of civil war for the putting down of rebellion,28 and the repelling of invasions to include the providing against an attempted or threatened invasion.29 The President may, when calling upon the militia, apply to the governors of the States to give the necessary orders, or may issue his orders directly to the commanding officers of the militia.30 When called into the federal service, the militia comes under the same complete federal control as the regular national forces, and of course subject to the rules and articles of war.31

In Martin v. Mott32 the doctrine was declared, which has not since been questioned, that the President is the sole and exclusive judge as to whether an exigency has arisen calling for a use of the militia by the federal authorities.