|Chain of command|
|army group||400,000–1,000,000||field marshal|
|army region||1,000,000–3,000,000||field marshal|
A field army or numbered army, usually referred to simply as an Army, is a military formation in many national military forces, superior to a corps and beneath an army group. Likewise, air armies are equivalent formation within some air forces.
Particular field armies are usually named or numbered to distinguish them from "army" in the sense of an entire national land force. In English, the normal style for naming field armies is word numbers, such as "First Army"; whereas corps are usually distinguished by Roman numerals (e.g. I Corps) and subordinate formations with ordinal numbers (e.g. 1st Division).
The Roman Army was among the first to feature a formal field army, in the sense of a very large, combined arms formation, namely the sacer comitatus, which may be translated literally as "sacred escort". The term is derived from the fact that they were commanded by Roman emperors (who were regarded as sacred), when they acted as field commanders. While comitatensis (plural: comitatenses) is sometimes translated as "field army", it may also be translated as the more generic "field force" or "mobile force" (as opposed to limitanei or garrison units).
In some armies, an "Army" is or has been equivalent to a corps-level unit in other armies. This was the case with a Gun (軍; "Army") within the Imperial Japanese Army, prior to 1945. The formation equivalent in size to a field army was an Area Army (方面軍; Hōmen-gun). Similarly, in the Soviet Red Army and the Soviet Air Forces, an Army was a corps-sized formation, subordinate in wartime to an Army Group-sized Front. It contained three to five maneuver divisions along with artillery, air defense, reconnaissance and other support formations. It could be classified as either a Combined Arms Army (CAA) or Tank Army (TA); while both were combined arms formations, the former contained a larger number of Motorized Rifle Divisions while the later contained a larger number of Tank Divisions. In peacetime, a Soviet army was usually subordinate to a military district.
Modern field armies are large formations which vary significantly between armed forces in size, composition, and scope of responsibility. For instance, within NATO a Field Army is composed of a headquarters, and usually controls at least two corps, beneath which are a variable number of divisions. A battle is influenced at the Field Army level by transferring divisions and reinforcements from one corps to another to increase the pressure on the enemy at a critical point. NATO Armies are controlled by a General or Lieutenant General.
See also 
- US Army, AR 840-10, Paragraph 5-11.
- US Army, FM 100-2-3 The Soviet Army: Troops, Organization and Equipment, Paragraph 1-3.