The history of the United States Army began in 1775, as part of the United States Armed Forces. The Army's main responsibility has been in fighting land battles and military occupation. The Corps of Engineers also has a major role in controlling rivers inside the United States. The Continental Army was founded in response to a need for professional soldiers in the American Revolutionary War to fight the invading British Army. Until the 1940s, the Army was relatively small in peacetime. In 1947, the Air Force became completely independent of the Army Air Forces. The Army was under the control of the War Department until 1947, and since then the Defense Department. The U.S. Army fought the Indian Wars of the 1790s, the War of 1812 (1812–15), Mexican–American War (1846–48), American Civil War (1861–65), American Indian Wars, Spanish–American War (1898), World War I (1917–18), World War II (1941–45), Korean War (1950–53) and Vietnam War (1965–71). Following the Cold War's end in 1991, Army has focused primarily on Western Asia, and also took part in the 1991 Gulf War and war in Iraq, and the war in Afghanistan.

The Continental Army was the army of the United Colonies representing the Thirteen Colonies and later the United States during the American Revolutionary War. It was formed on June 14, 1775 by a resolution passed by the Second Continental Congress, meeting in Philadelphia after the war's outbreak. The Continental Army was created to coordinate military efforts of the colonies in the war against the British, who sought to maintain control over the American colonies. General George Washington was appointed commander-in-chief of the Continental Army and maintained this position throughout the war.