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 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.
During the American Civil War, the United States Army, the land force that fought to preserve the collective Union of the states, was often referred to as the Union Army, the Federal Army, or the Northern Army. It proved essential to the restoration and preservation of the United States as a working, viable republic.
The Union army was made up of the permanent regular army of the United States, but further fortified, augmented, and strengthened by the many temporary units of dedicated volunteers, as well as including those who were drafted in to service as conscripts. To this end, the Union army fought and ultimately triumphed over the efforts of the Confederate States Army.
The Army of the United States was one of the four major service components of the United States Army. Today, the Army consists of the Regular Army, the Army National Guard of the United States, the Army National Guard while in the service of the United States, and the United States Army Reserve. Previously however the Army of the United States was a component. However, the Army of the United States has been inactive since the suspension of the draft in 1973 and the U.S. military's transition to an all-volunteer force. Personnel serving in the United States Army during a major national emergency or armed conflict (either voluntarily or involuntarily) were enlisted or inducted into the Army of the United States without specifying service in a component. It also includes the "Retired Reserve". Those are retired soldiers who have reached the required years of creditable service, or creditable service and age; regardless of the component, or components they formerly served in.
The term "Army of the United States" or "Armies of the United States" is also the legal name of the collective land forces of the United States as prescribed by the United States Constitution. In this concept, the term "Army of the United States" has been in use since at least 1841 as in the title General Regulations for the Army of the United States. Also included by Act of the 39th Congress (1866) were the U.S. Volunteers, persons holding a brevetted rank, and the U.S. Veteran Reserve Corps.