The task of organizing the U.S. Army commenced in 1775. During World War I, the "National Army" was organized to fight the conflict. It was demobilized at the end of World War I, and was replaced by the Regular Army, the Organized Reserve Corps, and the State Militias. In the 1920s and 1930s, the "career" soldiers were known as the "Regular Army" with the "Enlisted Reserve Corps" and "Officer Reserve Corps" augmented to fill vacancies when needed.

In 1941, the "Army of the United States" was founded to fight World War II. The Regular Army, Army of the United States, the National Guard, and Officer/Enlisted Reserve Corps (ORC and ERC) existed simultaneously. After World War II, the ORC and ERC were combined into the United States Army Reserve. The Army of the United States was re-established for the Korean War and Vietnam War and was demobilized upon the suspension of the Draft.

Currently, the Army is divided into the Regular Army, the Army Reserve, and the Army National Guard. The Army is also divided into major branches such as Air Defense Artillery, Infantry, Aviation, Signal Corps, Corps of Engineers, and Armor. Prior to 1903 members of the National Guard were considered state soldiers unless federalized by the President. Since the Militia Act of 1903 all National Guard soldiers have held dual status: as National Guardsmen under the authority of the governor of their state and as a reserve of the U.S. Army under the authority of the President.

Since the adoption of the total force policy, in the aftermath of the Vietnam War, reserve component soldiers have taken a more active role in U.S. military operations. Reserve and Guard units took part in the Gulf War, peacekeeping in Kosovo, and the 2003 invasion of Iraq.

Various State Defense Forces also exist, sometimes known as State Militias, which are sponsored by individual state governments and serve as an auxiliary to the National Guard. Except in times of extreme national emergency, such as a mainland invasion of the United States, State Militias are operated independently from the U.S. Army and are seen as state government agencies rather than a component of the military.

Although the present-day Army exists as an all volunteer force, augmented by Reserve and National Guard forces, measures exist for emergency expansion in the event of a catastrophic occurrence, such as a large scale attack against the U.S. or the outbreak of a major global war.

The final stage of Army mobilization, known as "activation of the unorganized militia" would effectively place all able bodied males in the service of the U.S. Army. The last time an approximation of this occurred was during the American Civil War when the Confederate States of America activated the "Home Guard" in 1865, drafting all males, regardless of age or health, into the Confederate Army.