Structure of the United States Army The Structure of the United States Army is complex , and able to be seen in several different ways : active / reserve , operational / administrative , and headquarters / field . This page aims to portray the current central structure of the US Army , perhaps in the future the former central structure ( including Army Ground Forces , Army Service Forces , and Army Air Forces ) and both present and historical combat formations ( armies , corps , and brigades ) . Active and Reserve . The United States Army is made up of three components : one active — the Regular Army ; and two reserve components — the Army National Guard and the Army Reserve . Both reserve components are primarily composed of part - time soldiers who train once a month , known as Battle Assembly or Unit Training Assemblies ( UTAs ) , and conduct two to three weeks of annual training each year . Both the Regular Army and the Army Reserve are organized under Title 10 of the United States Code . The National Guard is organized under Title 32 . While the Army National Guard is organized , trained and equipped as a component of the U . S . Army , individual units are under the command of individual state ' s governors . However , units of the National Guard can be federalized by presidential order and against the governor ' s wishes . During the First World War , the " National Army " was organized to fight the conflict . It was demobilized at the end of World War I , and was replaced by the Regular Army , the Organized Reserve Corps , and the State Militias . In the 1920s and 1930s , the " career " soldiers were known as the " Regular Army " with the " Enlisted Reserve Corps " and " Officer Reserve Corps " augmented to fill vacancies when needed . In 1941 , the " Army of the United States " was founded to fight the Second World War . The Regular Army , Army of the United States , the National Guard , and Officer / Enlisted Reserve Corps ( ORC and ERC ) existed simultaneously . After World War II , the ORC and ERC were combined into the United States Army Reserve . The Army of the United States was re - established for the Korean War and Vietnam War and was demobilized upon the suspension of the Draft . Currently , the Army is divided into the Regular Army , the Army Reserve , and the United States National Guard . Prior to 1903 members of the National Guard were considered state soldiers unless federalized by the President . Since the Militia Act of 1903 all National Guard soldiers have held dual status : as National Guardsmen under the authority of the governor of their state and as a reserve of the U . S . Army under the authority of the President . Since the adoption of the total force policy , in the aftermath of the Vietnam War , reserve component soldiers have taken a more active role in U . S . military operations . Reserve and Guard units took part in the Gulf War , peacekeeping in Kosovo , and the 2003 invasion of Iraq . Although the present - day Army exists as an all volunteer force , augmented by Reserve and National Guard forces , measures exist for emergency expansion in the event of a catastrophic occurrence , such as a large scale attack against the U . S . or the outbreak of a major global war . The final stage of Army mobilization , known as " activation of the unorganized militia " would effectively place all able bodied males in the service of the U . S . Army . The last time an approximation of this occurred was during the American Civil War when the Confederate States of America activated the " Home Guard " in 1865 , drafting all males , regardless of age or health , into the Confederate Army . Operational and Administrative . Headquarters Department of the Army ( HQ DA ) Staff . The U . S . Army is led by a civilian Secretary of the Army , who reports to the Secretary of Defense , and serves as civilian oversight for the U . S . Army Chief of Staff . The U . S . Army Chief of Staff is a member of the Joint Chiefs of Staff , a body composed of the service chiefs from each service who advise the President and Secretary of Defense on military matters under the guidance of the Chairman and Vice Chairman of the Joint Chiefs of Staff . Operational . Most U . S . Army units can be operationally divided into the following components from largest to smallest : Army Commands . The Army is currently undergoing a period of transformation , which is expected to be finished in 2013 . When it is finished , there will be six geographical commands which will line up with the five geographical Unified Combatant Commands ( COCOM ) . Each command will eventually have a numbered army as operational command , except in the case of U . S . Army Pacific , which will not maintain one but will have a numbered army for U . S . Army forces in the Republic of Korea . Army groups . Army groups were utilized during World War II only . Divisions . As the specialist page Divisions of the United States Army indicates , the US Army has had over 70 divisions in its twentieth century history . As an overview page , listing them all would distort the page . The compromise is to go " one back " and list the divisions as of peak strength during the Cold War , after the Reagan expansion had resulted in a force of 18 active and ten ARNG divisions . Regular Army Divisions ARNG Divisions Brigades . Combat Brigades : 44 Smaller units . Combat formations of the US Army at below brigade level include the United States Army Special Forces groups and several reserve separate battalions ( 100 – 442 Inf ( USAR ) , 3 - 172 Inf ( Mtn ) ( Vermont Army National Guard ) etc . ) . Arms and Services . < br > Personnel in the Army work in various branches . This term refers to their area of training or expertise . See : United States Army branch insignia . ( The term " Branch of service " also refers to the different United States armed forces . )