Army of the United States in the context of "Army Ground Forces"

The Regular Army of the United States succeeded the Continental Army as the country's permanent, professional land-based military force. In modern times, the professional core of the United States Army continues to be called the Regular Army (often abbreviated as "RA"). From the time of the American Revolution until after the Spanish–American War, state militias and volunteer regiments organized by the states (but thereafter controlled by federal authorities and federal generals in time of war) supported the smaller Regular Army of the United States. These volunteer regiments came to be called United States Volunteers (USV) in contrast to the Regular United States Army (USA). During the American Civil War, about 97 percent of the Union Army was United States Volunteers.

In contemporary use, the term Regular Army refers to the full-time active component of the United States Army, as distinguished from the Army Reserve and the Army National Guard. A fourth component, the Army of the United States, has been inactive since the suspension of the draft in 1973 and the U.S. armed forces became an all-volunteer armed force.

The Services of Supply or "SOS" branch of the Army of the USA was created on 28 February 1942 by Executive Order Number 9082 "Reorganizing the Army and the War Department" and War Department Circular No. 59, dated 2 March 1942. Services of Supply became one of the three autonomous components of the Army of the United States on 9 March 1942. It was renamed the Army Service Forces on 12 March 1943, as it was felt that the term "supply" did not accurately describe its broad range of activities. From the day of inception (and even before) in 1942 through the end of WWII, the SOS/ASF was commanded by Lieutenant General (later General) Brehon B. Somervell.

Most theaters of war had their own logistical organization, usually also named the Services of Supply. The European Theater, and its SOS was subdivided into the ETO and the MTO (Mediterranean Theater of Operations) for the Operation Torch invasion of North Africa, then Sicily, then Italy, though the MTO was largely supplied by the SOS out of Great Britain. The SOS-ETO became TSF/ET (Theater Service Forces—European Theater) on D-Day, 6 June 1944, the term SOS was abolished, and its activities on the continent were referred to as COM-Z, or Communications Zone.