The United States Army Reserve (USAR) is the federal reserve force of the United States Army. Together, the Army Reserve and the Army National Guard constitute the reserve components (RC) of the United States Army.

The Army Reserve was formed 23 April 1908 to provide a reserve of medical officers to the Army. After the First World War, under the National Defense Act on 4 June 1920, Congress reorganized the U.S. land forces by authorizing a Regular Army, a National Guard, and an Organized Reserve (Officers Reserve Corps and Enlisted Reserve Corps) of unrestricted size, which later became the Army Reserve.

Reserve soldiers perform only part-time duties as opposed to full-time (active duty) soldiers, but rotate through mobilizations to full-time duty. When not on active duty, reserve soldiers typically perform training/service one weekend per month, known as inactive duty for training (IADT) and currently referred to as Battle Assembly, and for two continuous weeks at some time during the year referred to as Annual Training (AT). Many reserve soldiers are organized into Army Reserve troop program units (TPU), while others serve in active Army units as Individual Mobilization Augmentees (IMA), or are in non-drilling control groups of the Individual Ready Reserve (IRR). Reserve Soldiers may also serve on active duty in support of the US Army Reserve (USAR) in an Active Guard/Reserve (AGR) status.

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.

A military reserve force is a military organization composed of citizens of a country who combine a military role or career with a civilian career. They are not normally kept under arms and their main role is to be available to fight when a nation mobilizes for total war or to defend against invasion. Reserve forces are generally not considered part of a permanent standing body of armed forces. The existence of reserve forces allows a nation to reduce its peacetime military expenditures while maintaining a force prepared for war. It is analogous to the historical model of military recruitment before the era of standing armies.

In some countries, such as the United States, Spain and the United Kingdom, members of the reserve forces are civilians who maintain military skills by training, typically one weekend a month. They may do so as individuals or as members of standing reserve regiments, for example the Territorial Army of the United Kingdom. In some cases a militia could constitute part of a military reserve forces, such as the United States National Guard. A Home Guard is a specific type of reserve force that can only be activated in the event of invasion.^{[citation needed]}