Militia Act of 1903 in the context of Militia in the United States

The militia of the United States, as defined by the U.S. Congress, has changed over time. During colonial America, all able-bodied men of a certain age range were members of the militia, depending on each colony's rule. Individual towns formed local independent militias for their own defense. The year before the U.S. Constitution was ratified, The Federalist Papers detailed the Founding Fathers' paramount vision of the militia in 1787. The new Constitution empowered Congress to "organize, arm, and discipline" this national military force, leaving significant control in the hands of each state government.

Today, as defined by the Militia Act of 1903, the term "militia" is used to describe two classes within the United States:

United States Volunteers (also called U.S. Volunteers, U.S. Volunteer Army, among other variations) were military volunteers called upon during wartime to help the United States Army. They were separate from the Regular Army and the militia.

Until the Militia Act of 1903, the land forces of the United States were divided into three organizations:

The National Defense Act of 1916, Pub. L. 64–85, 39 Stat. 166, enacted June 3, 1916, was a United States federal law that updated the Militia Act of 1903, which related to the organization of the military, particularly the National Guard. The principal change of the act was to supersede provisions as to exemptions. The 1916 act included an expansion of the Army and the National Guard, the creation of an Officers' and an Enlisted Reserve Corps, and the creation of a Reserve Officers' Training Corps. The President was also given expanded authority to federalize the National Guard, with changes to the duration and the circumstances under which he could call it up. The Army began the creation of an Aviation arm, and the federal government took steps to ensure the immediate availability of wartime weapons and equipment by contracting in advance for production of gunpowder and other material.