The structure of the United States Army is complex, and can be interpreted in several different ways: active/reserve, operational/administrative, and branches/functional areas.
From time to time the Department of the Army issues Department of the Army General Orders. In addition to base closures, unit citations, certain awards such as the Medal of Honor and Legion of Merit, they may concern the creation of JROTC units and structural changes to the Army. These are listed by year on the Army Publishing Directorate's website.
This page aims to portray the current overall structure of the US Army.
History
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 governors of their states 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.
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.
During World War I, the "National Army" was organized to fight the conflict. It was demobilized at the end of World War I.
After World War I, former units were 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.
Post World War II
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.
Active and reserve components
The United States Army is made up of three components: one active—the Regular Army; and two reserve components—the Army National Guard and the Army Reserve. Both reserve components are primarily composed of part-time soldiers who train once a month, known as Battle Assembly, Unit Training Assemblies (UTAs), or simply "drills", while typically conducting two to three weeks of annual training each year. Both the Regular Army and the Army Reserve are organized under Title 10 of the United States Code. The National Guard is organized under Title 32. While the Army National Guard is organized, trained, and equipped as a component of the U.S. Army, individual units are under the command of individual states' governors. However, units of the National Guard can be federalized by presidential order and against the governor's wishes.
Administrative
Headquarters Department of the Army (HQ DA) Staff
The U.S. Army is led by a civilian Secretary of the Army, who reports to the secretary of defense, and serves as civilian oversight for the Chief of Staff of the United States Army (CSA). The CSA is assisted by the Vice Chief of Staff of the United States Army. The CSA is a member of the Joint Chiefs of Staff, a body composed of the service chiefs from each service who advise the President of the United States and secretary of defense on military matters under the guidance of the chairman and vice chairman of the Joint Chiefs of Staff.