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 American military system developed from a combination of the professional, national Continental Army, the state militias and volunteer regiments of the American Revolutionary War, and the similar post-Revolutionary War American military units under the Militia Act of 1792. These provided a basis for the United States Army's organization, with only minor changes, until the creation of the modern National Guard in 1903. The Militia Act provided for the use of volunteers who could be used anywhere in time of war, in addition to the State militias who were restricted to local use within their States for short periods of time. Even today's professional United States Army, which is augmented by the Army Reserve and Army National Guard, has a similar system of organization: a permanent, professional core, and additional units which can be mobilized in emergencies or times of war.

The United States Army traces its origin to the founding of the Continental Army on June 14, 1775, when the Continental Congress authorized a one-year enlistment of riflemen from Pennsylvania, Maryland, and Virginia to join the New England militia army besieging Boston. Late in 1776, Congress called for the Continental Army to serve for the duration of the war. The army was to consist of 88 battalions raised and equipped by the states, with officers appointed by the states. Appointment of officers actually continued to be a collaboration between Congress, the Commander in Chief, George Washington, and the states. The number of battalions was to be apportioned to the states according to their populations. While the initial number of battalions approached the authorized strength, by 1 January 1787 the Continental infantry was only able to maintain enough regiments for fifty battalions. During the Revolutionary War, battalions and regiments were essentially the same. By October 19, 1781, when a British army under General Cornwallis surrendered to the American and French forces at Yorktown, the Continental Army had grown to sixty battalions.

For varying short periods of time during the war, many state militia units and separate volunteer state regiments (usually organized only for local service) supported the Continental Army. Although training and equipping part-time or short-term soldiers and coordinating them with professionally trained regulars was especially difficult, this approach also enabled the Americans to prevail without having had to establish a large or permanent army.

As the war waned, General Washington sent his plans for a standing army and organized militia to Congress. But due to the inability of Congress to raise much revenue under the Articles of Confederation, suspicion of standing armies, and perceived safety from foreign enemies provided by large oceans effectively controlled by the then non-hostile Royal Navy, Congress disbanded the Continental Army after the Treaty of Paris, the peace treaty with Great Britain, became effective. Congress retained 80 caretaker soldiers to protect arms and equipment at West Point, New York and Fort Pitt and called on the States to furnish 700 men from their militias for one year of service on the frontier. The delegates to the Constitutional Convention in Philadelphia in 1787 recognized the need for a more permanent military establishment and provided for a national regular army and navy and a militia under state control, subject to civilian control through congressional control of appropriations and presidential leadership as commander in chief of the regular forces and of the militia when called into federal service.

On June 3, 1784, the day after the Continental Army was reduced to 80 men, the Congress established a regiment which was to be raised and officered by obtaining volunteers from the militia of four of the states. This unit, the First American Regiment was commanded until 1 January 1792 by Josiah Harmar of Pennsylvania, gradually turned into a Regular regiment known as the 1st Infantry in 1791, and in 1815 was it redesignated as the 3rd Infantry in the reorganization of the army following the War of 1812. Congress gradually increased the military establishment from 700 men in 1784 to 5,104 in 1793.

The United States military realized it needed a well-trained standing army following St. Clair's Defeat on November 4, 1791, when a force led by General Arthur St. Clair was almost entirely wiped out by the Northwestern Confederacy near modern Fort Recovery, Ohio. The plans, which were supported by U.S. President George Washington and Henry Knox, Secretary of War, would lead to the creation of the Legion of the United States. The command would be based on the 18th-century military works of Henry Bouquet, a professional Swiss soldier who served as a colonel in the British Army, and French Marshal Maurice de Saxe.