The United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services. The modern Army has its roots in the Continental Army which was formed on 14 June 1775, before the establishment of the United States, to meet the demands of the American Revolutionary War. The Congress of the Confederation officially created the United States Army on 3 June 1784 after the end of the Revolutionary War to replace the disbanded Continental Army. The Army considers itself to be descended from the Continental Army and thus dates its inception from the origins of that force.

The primary mission of the army is "to fight and win our Nation’s wars by providing prompt, sustained land dominance across the full range of military operations and spectrum of conflict in support of combatant commanders." The Army is a military service within the Department of the Army, one of the three military departments of the Department of Defense. The army is headed by the Secretary of the Army, and the top military officer in the department is the Chief of Staff of the Army. The highest ranking army officer is currently the Chairman of the Joint Chiefs of Staff. During fiscal year 2010, the Regular Army reported a strength of 561,979 soldiers; the Army National Guard (ARNG) reported 362,015 and the United States Army Reserve (USAR) reported 205,281 putting the combined component strength total at 1,129,275 soldiers.

The United States of America (commonly abbreviated to the United States, the U.S., the USA, America, and the States) is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states and Washington, D.C., the capital district, lie between the Pacific and Atlantic Oceans, bordered by Canada to the north and Mexico to the south. The state of Alaska is in the northwest of the continent, with Canada to the east and Russia to the west, across the Bering Strait. The state of Hawaii is an archipelago in the mid-Pacific. The country also possesses several territories in the Pacific and Caribbean.

At 3.79 million square miles (9.83 million km²) and with over 312 million people, the United States is the third or fourth largest country by total area, and the third largest by both land area and population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries. The U.S. economy is the world's largest national economy, with an estimated 2011 GDP of $15.1 trillion (22% of nominal global GDP and over 19% of global GDP at purchasing-power parity). Per capita income is the world's sixth-highest.