From its formation in 1775, the United States Army has been the primary land based portion of the United States military. The Army’s primary responsibility has been the fighting of land battles and military occupation. The Corps of Engineers has a major role in controlling rivers. It was founded in response to a need for professional soldiery in the American Revolutionary War in order to fight the invading British Army. The Army was quite small in peacetime down to the 1940s. The Air Force became practically independent in World War II, and completely so in 1947. The Army was under the control of the War Department until 1947, and the Defense Department since then. The U.S. Army fought the War of 1812 (1812–15), American Civil War (1861–65), Spanish-American War (1898),World War I (1917–18), World War II (1941–45), Korean War (1950–53) and Vietnam War (1965–71). After the Cold War ended in 1991, the Army has focused on insurgencies in the Middle East, such as the Gulf War, and the recent War in Iraq and the War in Afghanistan.

When the American Revolutionary War began in April 1775, the colonial revolutionaries did not have an army. Previously, each colony had relied upon the militia, made up of part-time citizen-soldiers. The initial orders from Congress authorized ten companies of riflemen. The first full regiment of Regular Army infantry, the 3rd Infantry Regiment was not formed until June 1784.[1] After the war the Continental Army was quickly disbanded as part of the American distrust of standing armies, and irregular state militias became the new nation’s sole ground army, with the exception of a regiment to guard the Western Frontier and one battery of artillery guarding West Point‘s arsenal.

The War of 1812, an invasion of Canada failed, and U.S. troops were unable to stop the British from burning the new capital of Washington, D.C.. However, the Regular Army, under Generals Winfield Scott and Jacob Brown, proved they were professional and capable of defeating a major invasion by the regular British army in the Niagara campaign of 1814. Between 1815 and 1860, the main role of the Army was control of Indians in the West, and manning coast artillery stations at major ports. The U.S. used regular units and many volunteer units to fight Mexico, 1846–48. At the outset of the American Civil War the regular U.S. army was small and generally assigned to defend the nation’s frontiers from attacks by Native Americans. Following the Civil War, the U.S. Army fought a long battle with Native Americans, who resisted U.S. expansion into the center of the continent.

A combined conscript and volunteer force, the National Army, was formed by the United States War Department in 1917 to fight in World War I. During World War II, the Army of the United States was formed as a successor to the National Army. The end of World War II set the stage for the East-West confrontation known as the Cold War. With the outbreak of the Korean War, concerns over the defense of Western Europe rose. During the Cold War, American troops and their allies fought Communist forces in Korea andVietnam (see Domino Theory). The 1980s was mostly a decade of reorganization. The Army converted to an all-volunteer force with greater emphasis on training and technology. By 1989, the Cold War was coming to a close. The Army leadership reacted by starting to plan for a reduction in strength. After Desert Storm, the Army did not see major combat operations for the remainder of the 1990s. After the September 11 attacks, and as part of the Global War on Terror, U.S. and NATO combined arms (i.e. Army, Navy, Air Force, Marine, Special Operations) forces invaded Afghanistan in 2001, replacing the Taliban government. The Army took part in the combined U.S. and allied 2003 invasion of Iraq.