ADP 3-90, page 39

Common Tactical Concepts and Echelons 31 July 2019 ADP 3-90 2-21 exercises operational control over two or more Army divisions and a variety of multifunctional and functional supporting brigades. It exercises tactical control over various Marine Corps units and multinational units, and it is supported by various theater military intelligence, signal, and sustainment organizations and joint combat support agencies. The corps has both operational and administrative responsibilities. 2-105. A corps receives capabilities and units from the theater army to conduct operations. There is no standard organizational structure for a corps. However, a corps generally requires a maneuver enhancement brigade, a combat aviation brigade, an expeditionary sustainment command, a field artillery brigade, and a theater military intelligence brigade to conduct large-scale combat operations. Other units may provide direct or general support. 2-106. Based on the tasks of the divisions and the allocation of brigades, the corps commander determines the appropriate command and support relationships for subordinate divisions and brigades. A corps may retain a division or some number of brigades in reserve or for consolidation of gains activities. (However, an exception to this is that field artillery brigades are not retained in reserve or used for consolidation of gains activities.) FIELD ARMIES 2-107. A field army is an echelon of command that employs multiple corps, divisions, multi-functional brigades, and functional brigades to achieve objectives on land. The field army is the only echelon above the brigade that is only a headquarters. Large-scale combat operations involving multiple corps may require the establishment of a field army to function as the senior tactical or operational-level command exercising command and control over multiple corps-sized formations. The field army’s primary role is to serve as the ARFOR or joint force land component command for multi-corps operations. When serving as the joint force land component command, field armies require significant augmentation from the joint and multinational force. They are most likely to be employed in theaters where peer and near-peer adversaries have the capability of conducting large-scale ground combat. Field armies possess no standardized force structure. The Army tailors each field army to the conditions prevailing in its assigned area of responsibility. The field army receives the necessary capabilities and units from external sources based on its requirements.