United States Volunteers also known as U.S. Volunteers, U.S. Volunteer Army, or other variations of these, were military volunteers called upon during wartime to assist the United States Army but who were separate from both the Regular Army and the militia.
Previously to the enactment of the Militia Act of 1903, the land forces of the United States were divided into three separate and distinctive organizations.
The term Volunteers were first used in the Act of May 28, 1789, during the Northwest Indian War, which authorized the President of the United States to accept companies of volunteers who offered themselves for service. This and further enactments the president to organize the companies into legions, regiments or battalions, and to appoint company grade and field officers. Volunteer forces are next mentioned during the Quasi-War, when the Act of March 2, 1799, which authorized the president to accept 28 volunteer regiments to be employed in the same manners as the militia; the volunteer field officers to be appointed by the president with the advice and consent of the Senate of the United States. During the troubles with Spain in 1803, the president was, through Act of March 3, 1803, authorized to require the several states to organize forces of militia, including corps of volunteers. Trouble with Great Britain led to the enactment of two acts, the Act of February 24, 1807 and the Act of March 30, 1808, empowering the president to require the several states to organize forces of militia, including volunteer forces.
An act of January 12, 1812 authorized the President to raise up to six companies of rangers, either volunteers or men enlisted for a one-year period, whenever he had evidence of actual or threatened invasion of any Indian tribes. In July an additional company was authorized, and in February 1813 ten additional companies. 
On February 6, 1812 the Congress enacted the Volunteer Military Corps Act. This act provided for the raising of a large force, up to 50,000 soldiers, for a period of 12 months. This was a force directly under the Federal government; it was not militia and not under control of the several states.
The act of February 6,specified that the president were authorized to accept into federal service any volunteer company or companies of infantry, cavalry and artillery. They were to be clothed on their own expense but armed and equipped by the federal government when called into actual service. The cavalry were to provide their own horses. The commissioned officers were to be appointed according the laws of the several states. The volunteer units were to be called into service within two years after having been accepted and were then to serve for a 12 month period. While in actual service they were entitled to the same rules and regulations and receiving the same emoluments as the United States Army. In addition non-commissioned officers and men were to receive in money the cost of clothing themselves. Loss of horses and equipment furnished at their own expense were to be compensated. The president would organize the volunteer force into battalions, regiments, brigades and divisions as the case may be. Through the act of July 6, 1812 the right to appoint commissioned officers of the volunteers was given to the president with the advice and consent of the senate.
An exact count of the number of soldiers enlisted in the volunteer forces under the above act was never made. The most generally accepted figures are 3,049 volunteer rangers and 10,110 other federal volunteers. The act of February 6, 1812 was not successful; field officers were not appointed until sufficient number of companies had been formed into regiments. Companies from different states were never trained together before active operations.
In 1818, during the First Seminole War, General Edmund P. Gaines authorized Colonel David Brearley of the 7th U.S. Infantry to receive into the service of the United States Indians of the Creek Nation, to muster, inspect and provision them, and to order them to march to Fort Scott. Colonel Brearley met the Creek Nation at the Creek Agency and finding that nearly the whole force of the Nation was willing to serve began organizing them. The whole force was to be designed a regiment, with 18 companies, later 28 companies officered according to the regulations of the United States Army. The principal war chief, William McIntosh, to be full colonel; the two chiefs George Lovett and Noble Kennard to be majors. On February 24, 1818, 1,547 warriors entered the service of the United States and served until the beginning of May, the same year. Including Chief Onir Haujo and 75 warriors mustered into service on December 8, 1817, the whole Creek volunteer force 1,613 men.
William McIntosh held the rank of brigadier general. George Lovett, Noble Kennard, Samuel Hawkins and - Blue war majors. On April 18, by order of General Jackson Lovett and Kennard were promoted to full colonels, and Hawkins and Blue to lieutenant colonels. Two chiefs, John Bernard and - Mattey, who had served as captains, were promoted to majors. The staff further consisted of 1 assistant adjutant general and 4 assistant commissaries of purchase. The companies had 28 captains, 29 first lieutenants and 28 second lieutenants. These latter officers were appointed by the Creek Nation under the sanction of the commanding general.
Andrew Jackson authorized the raising of volunteers from Tennessee and Kentucky, and 1,286 men were mustered and organized as two mounted rifle regiments. The field and staff consisted of one assistant adjutant general, one assistant inspector general, one assistant deputy quartermaster general, one judge advocate, one chaplain, two colonels, four lieutenant colonels, four majors, four adjutants, one forage master, one assistant forage master, two surgeons, four surgeon's mates, four quartermasters and eight non-commissioned staff. The companies had 20 captains, 20 first lieutenants, 20 third lieutenants, 11 third lieutenants and 17 cornets. These troops served from January 31, 1818 to June 25, the same year.
Two companies of volunteer rangers of 145 men under Captains Boyle and Gist were also mustered into United States service by order of General Jackson. They were to be employed on search-and-destroy patrols between the Mobile and Appalachicola rivers. 
At the time of the Black Hawk War, United States Army lacked cavalry due to downsizing of the army after the War of 1812. The opening of the Santa Fe Trail led to demands for military escorts of the annual trading caravans across the prairies. In 1829 four infantry companies from Fort Leavenworth were ordered to protect that year's caravan. This expedition demonstrated the inferiority of foot soldiers against mounted Comanches. At the end of 1831, Senator Thomas Hart Benton of Missouri put forward a bill authorizing President Jackson to organize a mounted ranger unit of volunteers for frontier defense. The outbreak of the Black Hawk War meant that the bill was promptly passed by both houses of the Congress. 600 mounted rangers were to be enlisted, for a period of one year. The decision to organize a volunteer battalion instead of a regular cavalry regiment, emanated from the prevalent attitudes of the ruling Democratic Party toward the United States Army. The Regular Army was seen as a stronghold of aristocratic West Pointers in contrast with the virtuous citizen soldiers of the militia.
At the beginning of the Florida War or Second Seminole War, Congress in 1836 authorized the President to accept 10,000 volunteers. The militia of Florida Territory and of adjacent states had then already been called out. Congress prescribed that the volunteers should serve either as infantry or cavalry for a period of sex or twelve months, furnishing their own clothes and their own horses (if serving in the cavalry). Arms and equipment would be provided by the federal government. Officers were to be appointed according to the laws of states or territories in which the volunteer units were raised, although if already organized military units tendered their volunteer service they would be officered by the same officers as before volunteering.
Rules for pay and other emoluments of the volunteers in federal service provided that volunteers (and militia in federal service) would receive the same monthly pay, rations, clothing or money in lieu of clothing, forage, and travel allowance as offices and men of the United States Army. The would also be furnished with the same camp equipage, including knapsacks as the regular army. Officers and men of cavalry would receive 40 cents per day for use and risk of private horses.
When the number of men in volunteers unit fell below effective strength, new volunteer organizations were raised to take their place. Many of the volunteer units were engaged for so short periods that their service were inefficient and expensive. The daily allowance of 40 cents for horses made cavalry very costly. In addition the government had to pay for horses that died for lack of forage. About half of the volunteers serving in Florida were cavalry. About 10,000 regulars and up to 30,000 short-term volunteers had served in the Second Seminole War.
The Mexican War brought a major shift in the national military policy of the United States, namely the replacement of the militia system - "the great bulwark of the national defense" - with the volunteer system. The war of 1812 had been fought with the bulk of the soldiers coming from the militia. For the Mexican War, the United States mobilized 116,000 soldiers. Of them, a total of 42,000 served in the Regular Army, 13,000 in the Militia and 61,000 in the Volunteers. Of the volunteers, only about 30,000 served in Mexico.
The declaration of war with Mexico, May 13, 1846, was followed by large military appropriations, an increase in the regular army, and the authorization of the President to raise 50,000 volunteers for a term of one year or the duration of the war. The volunteers would serve either as cavalry, artillery, infantry or riflemen, furnish their own clothing and (for the cavalry) their own horses and horse equipment, but be armed by the government. They would serve under the articles of war, and receive all the emoluments of the Regular Army, except clothing, for which the enlisted men would receive compensation, and in addition 40 cent per day for the risk of the horses furnished. The volunteers would be organized in companies, battalions or regiments before volunteering; the officers appointed according to the laws of the several states that offered volunteers. The President was authorized to organize the volunteers into brigades and divisions if required, and appoint the staff, field and general officers among the several states.
The states closest to Mexico were asked to immediately provide 20,000 one-year volunteers, other states to have 25,000 ready for later call; about one-third of the volunteer units to be cavalry. The state quotas were easily filled. Volunteer units were much more easily filled than the increase in the Regular Army also authorized by Congress. The short-term enlistment and the easy discipline of the volunteers won out in comparison with the Regular Army's five-year enlistment and strict discipline. Thirty regiments of one-year volunteers were mustered, but in November 1846 it was obvious that their enlistment would expire before the end of the war. The President issued a call for volunteers to serve for the duration of the war and at the end of 1847, 22 regiments and 5 battalions of infantry, 1 regiment and 5 battalions of mounted troops and 3 companies of artillery had been organized. Several regiments were mustered later, making a total of 32 regiments "for the duration". By the end of May 1847, when the American army under Winfield Scott stood at Puebla, Mexico during its advance from Vera Cruz, the enlistment of the one-year volunteers in his army expired and seven volunteer regiments of 3,700 soldiers departed for home. The army had to halt and for two months wait for fresh troops from the states.
During the whole war, regulars and volunteers showed a marked degree of antipathy towards each others. Regular officers did not serve in the volunteers. Although the volunteers had excellent field officers they had but very few competent company officers. Most of the junior officers had no military experience or very little. They were either commissioned by the state governors for political reasons or elected by the enlisted men of the company. The reverse was true for the Regular Army, where few of the field officers were trained at West Point and many were non-effective through old age or infirmity. At Matamoros in 1846, about two thousand "gentlemen" who had enlisted as private volunteers mutinied because they had to draw water and chop wood; something they expected the Regular Army to do for them.
The following is an excerpt from GENERAL ORDERS No. 15., WAR DEPARTMENT, Adjutant General's Office, Washington, May 4, 1861 (links added):
The President of the United States having called for a Volunteer Force to aid in the enforcement of the laws and the suppression of insurrection, and to consist of thirty-nine regiments of infantry and one regiment of cavalry, making a minimum aggregate of (34,506) thirty-four thousand five hundred and six officers and enlisted men, and a maximum aggregate of (42,034) forty two thousand and thirty-four officers and enlisted men, the following plan of organization has been adopted, and is directed to be printed for general information:
The following is an excerpt from GENERAL ORDERS, No 126., WAR DEPARTMENT, Adjutant General's Office, Washington, September 6, 1862:
Each state was given a quota of "volunteer regiments" to be raised for service lasting from three months to three years, with quotas apportioned among the States according to population (see Military leadership in the American Civil War and American Civil War).
GENERAL ORDERS No. 30, HEADQUARTERS OF THE ARMY, ADJUTANT GENERAL' S OFFICE, Washington, April 30, 1898 reads, in part (links added):
I The following acts of Congress and Proclamation by the President are published for the information and government of all concerned:
An Act To provide for temporarily increasing the military establishment of the United States in time of war, and for other purposes.
Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That all able-bodied male citizens of the United States, and persons of foreign birth who shall have declared their intention to become citizens of the United States under and in pursuance of the laws thereof, between the ages of eighteen and forty-five years, are hereby declared to constitute the national forces, and, with such exceptions and under such conditions as may be prescribed by law, shall be liable to perform military duty in the service of the United States.
SEC. 2. That the organized and active land forces of the United States shall consist of the Army of the United States and of the militia of the several States when called into the service of the United States: Provided, That in time of war: the Army shall consist of two branches which shall be designated, respectively, as the Regular Army and the Volunteer Army of the United States.
The law provided for a presidential call for two-year volunteers, with quotas apportioned among the States according to population, and that militia units volunteering as a body had to be accepted as units into the Volunteer Army.